Light

Question 1

3 main properties of light waves:

Answer 1

• Light is a transverse electromagnetic wave
• Travels at 3 x 10^8 in a vacuum
• Travels in straight lines as a ‘ray’ until it hits a surface

Question 2

What is the normal?

Answer 2

Construction line which is drawn at 90 degrees to the surface where the ray hits

Question 3

What does a periscope use?

Answer 3

2 mirrors tilted at 45 degrees

Rays turn through 90 degrees twice
2 mirrors mean any left-right inversion is cancelled out

Question 4

What happens when light travels through a different transparent material

Answer 4

It travels slower as the material is denser

Question 5

What happens when light enters a glass block

Answer 5

It bends towards the normal

Question 6

What happens when light leaves a glass block

Answer 6

It bends away from the normal

Question 7

What is refraction?

Answer 7

The bending of the direction of travel of a wave as it moves from substance into another

Question 8

Investigating refraction practical:

Answer 8

1) place glass block on paper, draw around block
2) take a ray box and aim the box so that a single ray of light passes through the block
3) mark some points along the path of the ray
4) repeat the above process for different angles for the ray striking the block

Question 9

What happens when waves go from deep to shallow water?

Answer 9

Wavelength decreases

Question 10

Equation for refractive index

Answer 10

n = sin i / sin r

Question 11

How to measure the refractive index?

Answer 11

1) gradient = rise / run

2) calculating sin i / sin r for several points and taking an average

Question 12

Facts about refractive index:

Answer 12

• refractive index is a unit less quality
• a material with a greater refractive index will refract a ray of light more
• always greater than 1
• always give to 2dp

Question 13

How is the critical angle shown?

Answer 13

Using a semi - circular block

No refraction as light enters glass as it passes along the normal, when it leaves the back face it refracts

Question 14

What happens once the incident ray is passed 90 degrees in a semi-circular block

Answer 14

All of the light which hits the back face is reflected - total internal reflection

Question 15

What are the 2 conditions needed for total internal reflection?

Answer 15

1) light must be going from a more dense medium towards a less dense one
2) the angle of incidence must be greater than the critical angle

Question 16

Equation for critical angle and refractive index

Answer 16

Sin C = 1 / n

Question 17

What happens when light is sent into an equilateral prism

Answer 17

Light separates into visible spectrum - dispersion

At the first face, colours refract by different amounts
At second face, colours refract again, away from normal as they re- enter air

Question 18

What happens when white light enters into a right angled prism

Answer 18

If it enters the shorter sides along the normal - it doesn’t refract

If it hits a longer face, angle of incidence is 45 degrees (larger than critical angle) - total internal reflection occurs and ray is deviated by 90 degrees

Question 19

What happens when 2 prisms are used in a periscope

Answer 19

Longer side along normal - doesn’t refract
Shorter side - total internal reflection occurs
When it hits other shorter side - undergoes TIR and ray is deviated by 180 degrees

Used in binoculars

Question 20

What are optical fibres

Answer 20

Very thin tube of flexible glass
Narrow - total internal reflection occurs, any ray of light will bounce down the fibre without any light emerging from side of glass

Question 21

Uses of optical fibres:

Answer 21

1) send signals at high speeds - internet/ communications links, use infra red pulses to send data
2) bundles of fibres can be used in endoscopes to see inside a human body

Question 22

What is the critical angle

Answer 22

The angle of incidence that provides an angle of refraction of 90 degrees

Question 23

What is total internal reflection

Answer 23

the complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.