SP5 - Light and the Electromagnetic Spectrum

Question 1

What is the law of reflection?

Answer 1

The angle of reflection (angle between reflected ray and normal) is always equal to the angle of incidence (angle between incident ray and normal).

Question 2

What are the two types of reflection?

Answer 2

Specular reflection - when light is reflected off a smooth surface (e.g. a mirror), it reflects evenly.
Diffuse reflection - when light is reflected off a rough surface, it scatters.

Question 3

What happens if the angle of incidence is lesser than the critical angle?

Answer 3

Some of the light is reflected, but most is refracted.

Question 4

What happens if the angle of incidence equals the critical angle?

Answer 4

The refracted light passes along the interface (boundary) of the glass block.

Question 5

What happens if the angle of incidence is greater than the critical angle?

Answer 5

Total internal reflection - the light is completely reflected inside the block.

Question 6

What is the critical angle?

Answer 6

The angle of incidence at which total internal reflection starts to happen.

Question 7

Core practical - investigating refraction

Answer 7

• place a piece of plain paper on the desk, set up the power supply, ray box (single slit) and shine a ray of light across the paper.
• place a rectangular glass block on the paper, draw around it
• shine a ray of light into the block, use small crosses to mark where the rays of light go
• take the block off the paper, use a ruler to join the crosses to show the path of the light
• measure the angles of incidence and refraction where the light entered and left the block
• repeat this at different angles

Question 8

Why does a yellow object look yellow?

Answer 8

It reflects yellow light and absorbs all other colours.

Question 9

What is a filter?

Answer 9

A transparent material that absorbs some colours of light and transmit others. For example, a blue filter transmits blue light but absorbs all other colours.

Question 10

What colour will a red object appear when looked at through a green filter?

Answer 10

Black. This is because only green light is transmitted by the green filter and the red objects absorb all colours except for red, so it will absorb the green light and appear black.

Question 11

What is the power of a lens?

Answer 11

The amount it bends light that passes through it. The more it bends it, the more powerful a lens it is.

Question 12

What is a convex (converging) lens?

Answer 12

A lens which is fatter in the middle and thinner at the edges. It makes rays of light converge at the focal point.

Question 13

What is a concave (diverging) lens?

Answer 13

A lens which is thinner in the middle and thicker at the edges. It spreads rays out and the focal point is the point from which the rays seem to be coming after passing through the lens.

Question 14

What is a focal length?

Answer 14

The distance between the focal point and the centre of the lens.

Question 15

What happens if an object is viewed through a convex lens and is more than one focal length away?

Answer 15

A real image is formed (it can be projected into a screen). The image is inverted (upside down) and smaller than the object.

Question 16

What happens if an object is viewed through a convex lens and is less than a focal length away?

Answer 16

A virtual image will be formed (it cannot be projected onto a screen). The image appears to be on the same side as the object, and is upright and magnified. Magnifying glasses are convex lenses.

Question 17

What type of image do concave lenses form?

Answer 17

They always produce virtual images that are the same way up, much smaller and closer to the lens than the object.

Question 18

What is an electromagnetic (EM) wave?

Answer 18

EM waves are all transverse waves which travel at 3 x10^8 m/s in a vacuum and transfer energy.

Question 19

What makes up the EM spectrum?

Answer 19

Gamma X-ray UV visible light IR micro radio
————————————————————>
Increasing wavelength
Decreasing frequency

Question 20

How was infrared (IR) discovered?

Answer 20

Herschel wondered whether the different colours of light contain ‘different amounts of heat’. He split white light using a prism and put a thermometer in each of the colours. He also put a thermometer beyond the red section, where there was no visible light. He found that the ‘coldest’ colour was violet and the ‘hottest’ colour was red, also that ‘beyond red’ was hotter than the entire visible light spectrum. Herschel concluded that there must be something there to make it hot, and he called this infrared.

Question 21

What makes up the visible light spectrum?

Answer 21

Red Orange Yellow Green Blue Indigo Violet
————————————————————>
Increasing frequency
Decreasing wavelength

Question 22

What are some uses of visible light?

Answer 22

Light bulbs are designed to emit visible light. Cameras detect visible light and record images.

Question 23

What are some uses of infrared (IR)?

Answer 23

Short range communication (between computers in the same room or from a TV to its remote control unit). Fibre optic wires, grills and toasters, thermal imaging.

Question 24

What are some uses of microwaves?

Answer 24

Communications and satellite transmissions, mobile phone signals, cooking food in microwave ovens.

Question 25

What are some uses of radio waves?

Answer 25

Transmitting radio broadcasts and TV programmes, other communications (such as communicating with spacecraft).

Question 26

How are radio waves produced for broadcasting radio programmes?

Answer 26

Radio waves are produced by oscillations (variations in current and voltage) in electrical circuits. A metal rod or wire can be used as an aerial to receive radio waves. The radio waves are absorbed by the metal and cause oscillations in electrical circuits connected to the aerial, allowing radio stations to be picked up by radios.

Question 27

Why can radio waves be used for long range communication while microwaves can only be used for short range communication?

Answer 27

Waves travel in straight lines unless they are reflected or refracted. All microwaves are passed through the ionosphere, and so are not refracted. The curvature of the Earth prevents microwaves travelling a long distance. Some radio waves pass through the ionosphere, whilst others are refracted by it, meaning that they can reach much farther points than microwaves.

Question 28

What are some uses of ultraviolet (UV)?

Answer 28

Disinfecting water, sterilising dental/medical equipment, tanning beds, fluorescence used in security markings.

Question 29

What are some uses of X-rays?

Answer 29

Medical imaging (checking for fractures), scanning bags in the airport.

Question 30

What are some uses of gamma rays?

Answer 30

sterilise food and surgical instruments, radiotherapy, PET scans

Question 31

What are the dangers of non-ionising EM waves?

Answer 31

Radio - no harm
Micro - high frequency microwaves heat water in cells which could harm us as we are mostly water
IR - skin burns
Visible - eye damage from bright light

Question 32

What are the dangers of ionising EM waves?

Answer 32

UV - skin cancer, sunburns, DNA damage, cataracts/other eye damage
X-ray - cancers, cell mutations, DNA damage
Gamma - cancers, cell mutations, DNA damage

Question 33

How can something stay a constant temperature?

Answer 33

It must absorb the same amount of energy/power that it emits.

Question 34

Explain the greenhouse effect.

Answer 34

Some gases in our atmosphere (such as CO2) naturally absorb some energy. Humans have added more of these ‘greenhouse gases’, which has caused the Earth to radiate less power than it receives (as some is absorbed by the extra gases). Over time, the Earth will start radiating the same amount it receives again, but this time it will have a higher constant temperature than before.

Question 35

Core practical - investigating radiation

Answer 35

• cover four or more boiling tubes in different coloured materials
• pour the same volume of hot water from a kettle into each tube
• insert a bung with a thermometer into each tube, measure the temperature of the water in each tube and start the stop clock
• record the temperature of the water every 2 minutes for 20 minutes