Refraction🌈

Question 1

Explain dispersion?

Answer 1

When light goes through a prism it splits up into the colours of the rainbow. (This is called a spectrum).

Question 2

What causes dispersion?

Answer 2

Each of the colours are refracted in different amounts by the prism because they slow down different amounts when they get to the glass.

Question 3

What is meant by total internal reflection?

Answer 3

Total internal reflection occurs when the angle of incidence is greater than the critical angle and the ray of light is reflected (bounces back).

Question 4

What is the critical angle for glass/Perspex?

Answer 4

42 degrees exactly and always. After this angle, the light is totally internally reflected.

Question 5

Why are optical fibres made of 2 types of glass?

Answer 5

The cladding of optical fibres have a lower refractive index so are designed to reflect light back into the core without allowing it to escape.

Question 6

How do convex and concave lenses affect light?

Answer 6

• Convex lenses are thicker in the middle. This makes the parallel beams converge at the principal focus.
• Concave lenses are thinner in the middle. This makes the parallel beams diverge at the principal focus.

Question 7

Define the focal length.

Answer 7

The distance between the lens and the principal focus.

Question 8

Define the focal point.

Answer 8

A convex lens can focus light so it appears to meet at a single point - this is the focal point.

Question 9

Define the principal axis.

Answer 9

The middle ray of the light rays.

Question 10

Define the real image.

Answer 10

Something that the rays of light have to actually pass through.
Eg. The focus of a convex lens.
If it is an image, you have to be able to show it on a screen:
Eg. A projector.

Question 11

Define the virtual image.

Answer 11

The rays of light only look as though they came from it.
Eg. The focus of a concave lens.
If it is an image you can see it, but cannot show it on a screen:
Eg. The image in a plain mirror (flat mirror).

Question 12

What is refraction?

Answer 12

When light rays enter a more dense material (e.g. water or plastic) it slows down and moves towards the normal. When light enters a less dense material (e.g air) it speeds up and moves away from the normal.

Question 13

Name some examples of where refraction happens.

Answer 13

• The pencil half submerged in water-looks snapped in half because when light enters a more dense medium like water, it slows down and changes direction.
• The fish looks higher in water than it really is-the light rays are refracting when they leave the water because air is less dense than water.

Question 14

What causes refraction?

Answer 14

When the light ray enters a more dense medium it causes it to change speed. This change is speed causes it to change direction too and refract.

Question 15

What are the 2 conditions needed for total internal reflection to occur?

Answer 15

• The angle of incidence must be greater than the critical angle.
• In Perspex the critical angle must be 42 degrees. Any angle greater than this would result in all the light being reflected away.

Question 16

How do you make an accurate measurement of the critical angle for glass?

Answer 16

1. Place glass block in the middle of your page.
2. Draw an outline around it and find the middle of the straight edge.
3. Draw a 90 degree line through this point-this is the normal.
4. Direct a laser beam through the glass block.
5. Gradually make the angle of incidence larger until the angle of refraction is 90 degrees.

Question 17

Name 2 uses of optical fibres.

Answer 17

• Medical cameras.
• Electrical cables.