Topic 5 - Light and the Electromagnetic Spectrum

Question 1

5.1P - What is a ray diagram and how does it show reflection and refraction?

Answer 1

A ray diagram shows what happens to light when refracted or reflected.
In a ray diagram, the normal is a line drawn at right angles to the medium.
The angles of the incident and reflected ray are measured from the normal

Question 2

5.1P - What is the law of reflection?

Answer 2

When waves are reflected :
The angle of reflection = angle of incidence

Question 3

5.1P - What is refraction and reflection?

Answer 3

Refraction is when a light ray moves into a medium where it travels at a different speed and direction. Refraction doesn’t happen when light meets a medium at 90°.
Reflection is when a wave is bounced off the interface rather than being transmitted or absorbed.

Question 4

5.1P - What is total internal reflection?

Answer 4

When light passes from a more dense to a less dense medium, at small angles of incidence, the rays are refracted and some are reflected.
As the angle of incidence increases, so does the angle of refraction until the rays are refracted along the medium/interface.
If the angle of incidence increases more after this, the light is reflected in the medium, this is total internal reflection.
The point where the rays start reflecting internally is the critical angle.

Question 5

5.2P - What is the difference between specular and diffuse reflection

Answer 5

Materials with rough surfaces scatter light when reflected, this is diffuse reflection.
However, smooth surfaces reflect light evenly, this is specular reflection.

Question 6

5.3P - Why do surfaces have different colours?

Answer 6

White light is made from all the colours of the visible spectrum, it can be split into them using a prism.
When a white light ( sunlight ) hits a coloured object, it absorbs all the colours of the spectrum but reflects its colour, which humans see as the coloured object.
E.g. a yellow object looks yellow as it absorbs all the colours of the spectrum but reflects yellow.

Question 7

5.3P - How do filters make coloured light?

Answer 7

When a white light is shone at a transparent blue filter it transmits blue light while absorbing all the other colours.

Question 8

5.4P - What factors affect the power of a lens?

Answer 8

A lens is a piece of transparent material shaped to refract light.
The power of a lens is how much light is bent, the more powerful a lens the more light bent.
The more thicker a lens, the more light is bent so greater power.
Shorter the focal length ( length between focal point and the lens ), greater the power as light can be bent easier.

Question 9

5.5P - How do different shaped lenses refract light?

Answer 9

Converging lenses is fatter in the middle, making parallel rays converge at the focal point.
Diverging lenses are thinner in the middle, making the parallel rays spread out. Light comes from the focal point which is before the lens.

Question 10

5.6P - How do lenses produce real and virtual images?

Answer 10

Converging lenses focus light rays onto a screen. Images able to be projected on a screen are real images which are light rays that converge together.
Real images are inverted and smaller.
Converging lenses also form virtual images, they can’t be projected on a screen. It is upright and magnified like a magnifying glass.
Diverging lenses produce virtual images the same way up and smaller.

Question 11

5.7 - What do all electromagnetic waves have in common?

Answer 11

They are transverse and oscillate at right angles to the direction of energy transfer.
They travel at the same speed of 3 x 10^8 m/s in a vacuum.
They transfer energy from source to observer.

Question 12

5.8 - What are some examples of EM waves transferring energy from source to observer?

Answer 12

Microwaves - energy from source to food
Sun - energy from Sun to Earth

Question 13

5.9 - Core Practical : Refraction in a glass block with EM waves

Answer 13

Place a piece of paper. Set up a ray box.
Place the rectangular glass block on the paper and draw an outline of it.
Shine a ray of light in the block, marking where the rays enter and leave.
Use a ruler to join to markings and show the rays path.
Measure the angles of incidence and refraction where the ray entered and left.
Repeat until the light ray passes along the interface and then TIR happens.

Question 14

5.10 - What are the main groupings of the electromagnetic spectrum?

Answer 14

Gamma rays - shortest wavelength/highest frequency
X-rays
Ultraviolet rays
Visible light
Infrared rays
Microwaves
Radiowaves - longest wavelength/lowest frequency

Question 15

5.11/12 - What characteristics of EM waves are used to group them?

Answer 15

The EM spectrum is continuous, all wavelengths are possible, so they are grouped into higher frequency waves with shorter wavelength and lower frequency waves have longer wavelength.
Human eyes can’t see short or long wavelength waves, instead it sees visible light ( ROY G BIV ) which is between the two types of waves.

Question 16

5.13 - What are the differences in waves with different wavelengths based on how they absorb, transmit, refract or reflect EM waves?

Answer 16

Astronomers use telescopes to study EM waves, although different waves are used for different wavelengths, because different materials affect EM waves differently.
For example, the Earth’s atmosphere only allows some EM waves through and absorbs the others.
The type of material used in telescopes depends on the wavelength being studied.

Question 17

5.14 - Explain the effects of different velocities of EM waves in different substances

Answer 17

Substances more dense than air slow EM waves while substances like air are faster compared to it.

Question 18

5.15P - How does the radiation emitted by a body depend on its temperature?

Answer 18

As temperature increases, so does amount of radiation emitted.
As temperature increases, the wavelength of the radiation becomes shorter.

Question 19

5.16P/17P - How does the temperature of a body depend on how much power it absorbs and radiates?

Answer 19

For a system/body to stay at a constant temperature it must absorb and radiate the same amount of power(the amount of energy transferred in a certain time ).
When at a constant temperature, it reaches equilibrium.
If the average power radiated is less than it absorbs, overall, the system absorbs power leading to increased temperature of it.
If the average power radiated is more than it absorbs, overall, the system radiates power leading to decreased temperature of it.

Question 20

5.18P - How is the Earth affected by different factors?

Answer 20

The Earth absorbs half of the radiation that reaches it from the Sun.
It re-radiated this energy as infrared radiation which warms the atmosphere.
To keep the temperature constant to radiated and absorbed energy is equal.
Gases in our atmosphere absorb some of the infrared radiation keeping the Earth at a constant temperature.
This is the greenhouse effect and the gases are greenhouse gases.
Due to humans, scientists believe the Earth is warming up as there are increased greenhouse gases.
To decrease the temperature, the greenhouse gases should be removed.

Question 21

5.19P - Core Practical : How does the surface of an object affect the energy it radiates or absorbs?

Answer 21

Use 4 different coloured flasks (shiny silver, dull grey, shiny black and dull black)
Pour the same volume of boiling water in the flasks with a kettle.
Insert a bung with a thermometer in each tube.
Measure and record the starting temperature and at 2 minute intervals for 20mins.

Question 22

5.20 - How is the danger of EM waves linked to its frequency?

Answer 22

Higher frequency waves transfer more energy than low frequency waves so are dangerous.

Question 23

5.21 - What are the dangers of EM radiation?

Answer 23

All waves transfer energy.
Microwave heating is dangerous for humans as this frequency can heat our cells as they have water.
Infrared radiation is absorbed by the skin, felt as heat, too much infrared radiation can damage cells and cause skin burns.
Ultraviolet radiation can cause sunburn and damage DNA. Long-term exposure can lead to skin cancer. It can also damage eyesight as it damages the retina.
X-rays and gamma rays can penetrate the body with its short wavelength, prolonged exposure leads to mutations in the DNA leading to cells killed or cancer cells.

Question 24

5.22 - What are some uses of EM waves?

Answer 24

Radiowaves are used in transmitting broadcasts and TV programmes.
Microwaves are used in communications and satellite transmissions.
Infrared radiation is used in short range communication, toasters to transfer heat energy to the food and tripwire alarms.
Visible light is the waves humans detect and is used in bulbs and cameras.
Ultraviolet radiation can be used in fluorescence where it is absorbed and re-emit as visible light, an example of this is fluorescent bulbs.
X-rays can be used in medicine to observe the internal structure of humans and the internal structure of objects in scanners.
Gamma rays can kill cells, so can be used to sterilise food and surgical equipment by killing pathogens. They can kill cancer cells in radiotherapy and detect tumours by feeding it a gamma radiated glucose that can be scanned for its location.

Question 25

5.23 - How are radio waves produced and detected?

Answer 25

Radiowaves are produced by oscillations in electrical circuits.
A metal rod is used to receive aerial waves which are absorbed and cause oscillations to the electrical circuits in the rod.
Radiowaves can be refracted when travelling through a layer of the atmosphere called the ionosphere, where they could be refracted enough to return to Earth, where they are detected.
The ionosphere is a region of charged particles.

Question 26

5.24 - How is EM radiation linked to changes in atoms?

Answer 26

EM radiation is produced by changes in the nuclei of atoms, it could have changes in the way its arranged to make a type of radiation.
Radiation absorption can also produce ions and changes to nuclei.