SP5: Light and the Electromagnetic Spectrum

Question 1

SP5a
1) What is the law of reflection?
2) What is total internal reflection, and when does it happen?
3) What is the significance of the critical angle in total internal reflection?
4) What is the critical angle?

Answer 1

1) The law of refraction is that when waves are reflected, the angle of reflection is equal to angle of incidence.
2) Total internal reflection is when light is moving from a denser medium towards a less dense one, instead of being refracted, all of the light is reflected.
Total internal reflection (TIR) occurs when: The angle of incidence is greater than the critical angle and the incident material is denser than the second material.
3) At any angle greater than the critical angle, total internal reflection occurs and the light ray obeys the normal rules for reflection.
4) The critical angle is angle of incidence when the ray changes from just refracting to TIR.

Question 2

SP5b
1) What is the difference between specular and diffuse reflection?
2) What is white light?
3) Explain why surfaces appear to have different colours, in terms of differential absorption

Answer 2

1) Diffuse reflection is when the reflected light is scattered in all directions.
Specular reflection is when very smooth surfaces, such as mirrors, reflect the light evenly.
2) White light is a mixture of different colours of light.
3) When white light hits a coloured surface, some of the colours that make it up are absorbed and some are reflected. For example, a yellow object absorbs the colour yellow, but reflects all the other colours.

Question 3

SP5b
1) Explain how filters make coloured light in terms of absorption and transmission
2) Explain the effect of viewing coloured objects in different colours of light
3) Explain what would happen if two different coloured filters were put in front of each other, and white light was shone through it

Answer 3

1) Filters absorb most of the colours in white light, while transmitting only a certain colour. For example, a blue filter transmits (allows through) blue light and absorbs all the other colours. Therefore, white light will always appear the colour of the filter.
2) An object appears to be black if it absorbs all the wavelengths of visible light. For example, an object that appears blue in white light will appear black in red light. This is because the red light contains no blue light for the object to reflect.
3) No light would be able to pass through the filters, as all of the colours of light are absorbed.

Question 4

SP5c
1) Define the power of a lens
2) What is the focal length and describe how the focal length and shape of a lens affect its power
3) What is a converging lens and how does it refract light?

Answer 4

1) The power of a lens describes how much it bends light that passes through. A more powerful lens is more curved and bends the light more.
2) The focal length is the distance between the focal point and the centre of the lens. If a lens has a shorter focal length, then the lens is more powerful.
3) A converging lens is thicker in the middle than at the edges. It makes parallel rays of light converge (come together) at the focal point.

Question 5

SP5c
1) What is a diverging lens and how does it refract light?
2) What is the focal point?
3) What type(s) of image(s) is produced by a diverging lens?

Answer 5

1) A diverging lens (concave) is thinner in the middle than at the edges. This causes parallel rays to diverge. They separate, but appear to come from a focal point on the other side of the lens.
2) The focal point (also called the principal focus) is the point from which the rays seem to be coming after passing through the lens.
3) Diverging lenses always produce virtual images that are the same way up, much smaller and closer to the lens than the object.

Question 6

SP5c
1) Explain the different type(s) of image(s) that can be formed by converging lenses
2) What is a real image?
3) What is a virtual image?

Answer 6

1) A converging lens can form a real image or a virtual image.
2) A real image is an image formed where the light rays are focused to form an image, and can be projected onto a screen.
3) A virtual image is an image where light rays don’t come together where the image appears to be. A virtual image cannot be projected on a screen.

Question 7

SP5d
1) Recall examples of electromagnetic waves
2) Describe the common features of electromagnetic waves
3) Describe the transfer of energy by electromagnetic waves

Answer 7

1) Electromagnetic waves include visible light, ultraviolet rays and x-rays
2) All electromagnetic waves are transverse waves, and they all travel at the same speed in a vacuum.
3) Electromagnetic waves transfer energy from a source to an observer.

Question 8

SP5d
1) Describe the range of electromagnetic waves that our eyes can detect
2) Describe an effect caused by the different velocities of electromagnetic waves in different substances

Answer 8

1) The only electromagnetic waves that our eyes can detect is visible light. We can see all the colours from red to violet.
2) Refraction is caused by differences in the velocity of waves travelling through different substances.

Question 9

SP5e
1) Recall the groups of waves in the electromagnetic spectrum in order of increasing wavelength
2) Recall the colours of the visible spectrum in order
3) Describe how the waves in the electromagnetic spectrum are grouped

Answer 9

1) (Shortest wavelength) Gamma rays, x-rays, ultraviolet rays, visible light, infrared, micro-waves, radio waves (Longest wavelength)
2) Red, orange, yellow, green, blue, indigo, violet
3) The waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency.

Question 10

SP5e
1) Describe some differences in the ways that different parts of the electromagnetic spectrum are absorbed and transmitted by the body (radio waves, micro waves and ultraviolet)
2) Describe some differences in the ways that different parts of the electromagnetic spectrum are refracted and reflected by the body

Answer 10

1) - Radio waves are transmitted through the body without being absorbed
- Some wavelengths of micro waves can be absorbed, causing heating of cells which may be dangerous
- Ultraviolet is absorbed by the skin
2) Infrared and visible light are mostly reflected or absorbed by the skin which causes some heating (IR can cause burns if the skin gets too hot).

Question 11

SP5f
1) Describe how long wavelength electromagnetic waves are affected by different substances
2) Explain the effects caused by long wavelength electromagnetic waves travelling at different velocities in different substances
3) Describe some uses of radio waves (state 3)

Answer 11

1) Different substances may absorb, transmit, refract, or reflect electromagnetic waves in ways that change with wavelength.
2) Changing velocity can lead to refraction. When the density increases, the velocity of the waves decreases. When the density decreases, the velocity increases.
3) Radio waves are used for communication such as broadcasting television and radio, communications and satellite transmissions.

Question 12

SP5f
1) Describe some uses of microwaves (state 3)
2) Describe some uses of infrared (state 6)
3) Describe some uses of visible light
4) What is the approximate wavelength for a ray of visible light?

Answer 12

1) Microwaves are used for cooking food, communications and for satellite communications.
2) Infrared (IR) light is used for: cooking, thermal imaging, short range communications, optical fibres, television remote controls and security systems.
3) Visible light is used for: vision, photography and illumination.
4) A ray of visible light has a wavelength of proximately 500 nm (5 x 10⁻⁷ m).

Question 13

SP5f
Describe how radio waves are produced and detected by electrical circuits

Answer 13

An alternating current flows through the aerial. Radio waves are produced by oscillations in electrical circuits of the aerial. A metal rod or wire can be used as an aerial to receive radio waves. The material must be an electrical conductor, so a material such as plastic cannot be used as an aerial because plastic is not an electrical conductor. Radio waves are absorbed by the metal in the receiving aerial and cause oscillations in electric circuits connected to the aerial. This produces an alternating current in the second aerial.

Question 14

SP5g
1) Describe how the intensity and wavelength of emitted radiation depends on the temperature of the body
2) What is power, and how can a system stay at a constant temperature?
3) Explain what happens to the temperature of a body when the average power radiated is not balanced by the average power absorbed
4) Why does the temperature of the filament in an electric filament lamp rise and then stay constant?

Answer 14

1) The distribution of the emitted wavelengths of radiation is affected by temperature.
The intensity (amount) of radiation emitted by an object increases as its temperature increases. The higher the temperature, the shorter the wavelength.
At low temperatures, the intensity of radiation emitted is low and the range of emitted wavelengths of radiation are high.
2) Power is the amount of energy transferred in a certain time. For a system (eg. the body, or a lightbulb) to stay at a constant temperature (eg. body temperature, or the temperature of a lightbulb) , it must absorb the same amount of power as it radiates.
3) The body temperature will either be too high or too low, because there is not a balance between the average power being radiated and absorbed.
4) Initially, the filament is emitting less energy than is being supplied to it. The temperature rises until the energy emitted is equal to the energy supplied.

Question 15

SP5g
Explain how the temperature of the earth is maintained and how it is affected by other factors

Answer 15

• For Earth to maintain a constant temperature it must radiate the same average power as it absorbs.
• If it absorbs more power than it radiates, its temperature will increase.
• If it radiates more power than it absorbs, its temperature will decrease.
• Some gases in the atmosphere (such as carbon dioxide/greenhouse gases) absorb energy and so increase the temperature of the Earth. The more greenhouse gases there are, the more energy that is absorbed, increasing temperature of the earth.
• The atmosphere keeps the Earth warmer than if there was no atmosphere.
• The temperature increases until the average power absorbed and radiated are equal. Then, the temperature will remain the same.

Question 16

SP5h
1) Describe how short wavelength electromagnetic waves are affected by different substances
2) Explain the effects caused by short wavelength electromagnetic waves travelling at different velocities in different substances

Answer 16

1) Different substances may absorb, transmit, refract, or reflect electromagnetic waves in ways that change with wavelength.
2) Changing velocity can lead to refraction. When the density increases, the velocity of the waves decreases. When the density decreases, the velocity increases.

Question 17

SP5h
1) Describe some uses of ultraviolet radiation (state 3)
2) Describe some uses of X-rays (state 2)
3) Describe some uses of gamma rays (state 3)

Answer 17

1) Fluorescent substances are used in energy-efficient lamps, banknote security and disinfecting water.
2) X-rays can be used in security at airports, and in medicine to make images on inside the body. This is because x-rays can pass through muscles and fat due to its short wavelength. However, rays with a longer wavelength (such as ultraviolet) are stopped by the skin.
3) They are used to sterilise food and medical instruments, and detect and treat cancer.

Question 18

SP5i
1) Describe how the potential danger of electromagnetic radiation depends on its frequency
2) Describe the harmful effects of microwave and infrared radiation

Answer 18

1) Higher frequency waves transfer more energy than lower frequency waves, and so are potentially more dangerous.
2) Microwave radiation: Can be dangerous to people because our bodies are mostly water and so the microwaves could internally heat body cells.
Infrared radiation: Our skin absorbs infrared radiation, which we feel as heat. Too much infrared radiation can damage or destroy cells, causing burns to the skin.

Question 19

SP5i
1) Describe the harmful effects of ultraviolet radiation, X-rays and gamma rays
2) Explain the effect that the absorption of radiation can have on atoms.

Answer 19

1) Ultraviolet radiation: can cause damage to surface cells and eyes, leading to skin cancer and eye conditions.
X-rays and gamma rays: X-rays and gamma rays are higher frequency than ultraviolet radiation and so transfer more energy. Excessive exposure to X-rays or gamma rays may cause mutations or damage to cells in the body.
2) Absorption of radiation can cause an atom to lose an electron. This means that it causes the atom to become an ion.

Question 20

SP5g - Core Practical
1) What is the aim for the investigating radiation core practical?
2) What is the method for the investigating radiation core practical?

Answer 20

1) Investigate how the nature of a surface affects the amount of thermal energy radiated or absorbed.
2) A) Cover four or more boiling tubes in different coloured materials.
B) Pour the same volume of hot water from a kettle into each tube.
C) Insert a bung with a thermometer into each tube. Measure the temperature of the water in each tube and start a stop clock.
D) Record the temperature of the water in each tube every 2 minutes for 20 minutes.

Question 21

SP5g - Core Practical
1) What are the risks for th investigating radiation core practical?
2) What errors may occur for the investigating radiation core practical?

Answer 21

1) - Keep water away from all electrical equipment
- Make sure not to touch the hot water directly
- Do not overfill the kettle
2) - Make sure the starting temperature for the water is the same in each material
- Use a data logger connected to a digital thermometer to get more accurate readings

Question 22

SP5g - Core Practical
1) What types of paper are the best and worst emitters of radiation?
2) What are some ways to improve the investigating radiation core practical?

Answer 22

1) Shiny white paper is the worst emitter of radiation, and matt black paper is the best emitter of radiation. Therefore, the temperature of the water inside the shiny white paper test tube will stay warmer for longer.
2) You can take more frequent measurements of the temperature (eg. Every 30 seconds instead of 2 minutes), and use a digital thermometer.

Question 23

SP5a - Core Practical
1) What is the aim for the investigating refraction core practical?
2) What is the method for the investigating refraction core practical?

Answer 23

1) Investigate refraction in rectangular glass blocks in terms of the interaction of electromagnetic waves with matter
2) A) Place a piece of plain paper on the desk. Set up the power supply, ray box and single slit so that you can shine a single ray of light across the paper on your desk. Take care, as ray boxes can become very hot.
B) Place a rectangular glass block on the paper. Draw around the block.
C) Shine a ray of light into your block. Use small crosses to mark where the rays of light go.
D) Take the block off the paper. Use a ruler to join the crosses to show the path of the light, and extend the lines so they meet the outline of the block. Join the points where the light entered and left the block to show where it travelled inside the block.
E) Measure the angles of incidence and refraction where the light entered the block, and measure the angles where it left the block.
F) Repeat steps C to E with the ray entering the block at different angles.
G) Move the ray box so that the light ray reaches the interface at right angles. Note what happens to the light as it enters and leaves the block.

Question 24

SP5g - Core Practical
1) What are the risks for the for the investigating refraction core practical?
2) What errors may occur for the investigating refraction core practical?
3) What are the independent, dependent and control variables for the investigating refraction core practical?

Answer 24

1) - The ray box light could cause burns if touched, as it will be hot
- Looking directly into the light may damage the eyes
- Keep all liquids away from the electrical equipment and paper
2) - An error could occur if the 90° lines are drawn incorrectly, so use a set square to draw perpendicular lines
- The points for the incoming and reflected beam may be inaccurately marked, so use a sharpened pencil and mark in the middle of the beam
- The protractor resolution may make it difficult to read the angles accurately, so use a protractor with a higher resolution
3) Independent variable = shape of the block
Dependent variable = direction of refraction
Control variables:
Width of the light beam
Same frequency / wavelength of the light

Question 25

SP5c
1) How do you draw a convex (converging) lens ray diagram?
2) How do you draw a concave (diverging) lens ray diagram?

Answer 25

1) Start by drawing a ray going from the top of the object through the centre of the lens. (Some diagrams will have a line through centre of the lens, draw up to this line.) This ray will continue to travel in a straight line. Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly through the principal focus (f).
If the object is closer than the focal length, then the rays create a virtual image, and dashed Ines must be drawn beyond the image to show where the rays meet.
The image is the line drawn from the axis to the point where the above two rays meet.
2) Start by drawing a ray going from the top of the object through the centre of the lens. (Some diagrams will have a line through centre of the lens, draw up to this line.) This ray will continue to travel in a straight line. Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly upwards away from the axis. Draw a dashed line continuing this ray downwards to the focal point (f).
The image is the line drawn from the axis to the point where the above two rays meet.

Question 26

SP5f
1) How do microwaves cook food in a microwave oven?
2) How does an infrared camera work?
3) How do x-rays form medical images?
4) How is fluorescence used in security?

Answer 26

1) The microwaves are absorbed by water molecules in the food. This transfers energy to the water molecules, causing the water in the food to heat up. The water molecules transfer the energy that they have absorbed to the rest of the molecules in the potato, which cooks it.
2) The camera detects infrared radiation emitted by an object and converts this into an electrical signal which is displayed on a screen as an image. Different temperatures appear brighter or as different colours, so you can build a thermal image of the surroundings.
3) X-rays are directed at the body part being imaged. A detector is placed behind the body, The X-rays are absorbed by bones, but transmitted by the rest of the body tissue and muscles. A negative image is formed with brighter areas where fewer X-rays get through, indicating the bones. Ultraviolet waves cannot be used as they are partially reflected by the skin.
4) Fluorescence is used in marking bank notes. An ultraviolet light can be shone onto the bank note. The fluorescent ink absorbs ultraviolet light, and then emits visible light. This will make the fluorescent ink visible, and prove the authenticity of the bank note.

Question 27

1) What is the angle of incidence?
2) What is the angle of refraction?
3) What is the angle of reflection?

Answer 27

1) The angle of incidence is measured between the incoming light ray (the incident ray) and the normal. The normal is an imaginary line at 90° to the reflecting surface and is drawn as a dotted line.
2) The angle of refraction is the angle between the between the refracted ray (the light ray after passing through the boundary) and the normal.
3) The angle of reflection is measured between the reflected light ray and the normal. The angle of incidence = the angle of reflection.

Question 28

Explain how gamma rays are produced

Answer 28

Gamma rays may result from radioactive decay. Gamma rays are produced by energy changes, and rearrangement in the nucleus. Gamma rays are produced to stabilise the nucleus.