Light

Question 1

What type of waves are light waves?

Answer 1

light waves are transverse waves

Question 2

What three things can light waves be?

Answer 2

reflected

refracted

diffracted

Question 3

What allows us to see most objects?

Answer 3

reflection of visible light is what allows us to see most objects

light bounced off the objects into our eyes

Question 4

What happens when light reflects from an uneven surface?

Answer 4

when light reflects from an uneven surface such as a peice of paper, the light reflects off at all different angles and you get a diffuse reflection

Question 5

What happens when light reflects from an even surface?

Answer 5

when light reflects from an even surface (smooth and shiny like a mirror) then it’s all reflected at the same angle and you get a clear reflection

Question 6

What is the law of reflection?

Answer 6

the angle of incidence = the angle of reflection

Question 7

When are images formed?

Answer 7

images are formed when light waves cross

Question 8

Always measure from the … to the ray itself

Answer 8

Always measure from the Normal​ to the ray itself

Question 9

When is a real image produced?

Answer 9

a real image is produced when real rays of light cross

Question 10

When are virtual images produced?

Answer 10

a virtual image is produced when virtual rays of light cross

Question 11

What are the differences between a real image and a virtual image

Answer 11

a real image is produced when real rays of light cross and can be projected

a virtual image is produced when virtual rays of light cross and cannot be projected

Question 12

What is three things is virtual image compared to the real image?

Answer 12

the virtual image is:

the same size

laterally inverted

the same distance behind the mirror as the real object is in front

Question 13

What are our brains hard wired to think?

Answer 13

our brains are hardwired to think that light travels in straight lines

we can be fooled to think the image behind the mirror because our brains do not know that the real rays of light have been reflected of the mirror and have changed direction

Question 14

How do you construct ray diagram to illustrate the formation of a virtual image in a plane mirror?

Answer 14

1. Draw the virtual image. The image is the same size as the real image and is the same distance behind the mirror as the real object is in front
2. Draw a reflected ray going from the top of the virtual image to th top of your eye. Draw a bold line for the part of the ray between the mirror and eye, and a dotted line for the part of the ray between the mirror and the virtual image
3. Draw an incident ray going from the top of the onject to the mirror. The incident and reflected rays follow the law of reflection - but you don’t have to measure any angles. Draw the ray from the object to the point where the reflected ray meets the mirror
4. Now you have an incident ray and a reflected ray for the top of the image. Repeat steps 2 and 3 again for the bottom of the image - a reflected ray going from the image to the bottom of the eye, then an incident ray from the object to the mirror

Question 15

The normal is always …^o (parallel/perpendicular) to the mirror

Answer 15

The normal is always 90^o (perpendicular) to the mirror

Question 16

What is refraction?

Answer 16

refraction is the bending of light which occurs at the boundary of two different medium, due to a change of density which changes the speed of light - it occurs when light travels from one medium to another

‘changes direction’

Question 17

EM waves, therefore light waves, usually travel faster/slower in denser media

The light rays get closer together/farther apart

Answer 17

EM waves, therefore light waves, usually travel slower in denser media

The light rays get closer together

Question 18

Design an experiment to investigate the refraction of light using a retangular block of a particular material (e.g. glass) resting on top of a piece of paper. What happens?

Answer 18

1. Shine a light ray at an angle into the block. Some of the light is reflected, but a lot of it passes through the glass and gets refracted as it does so
2. Trace the incident and emergent rays onto the piece of paper and remove the block. You can draw in the refracted ray through the block by joining the ends of the other two rays with a straight line
3. You should see that as the light passes from the air into the block (a denser medium), it bends towards the normal. This is because it slows down
4. When the light reaches the boundary on the other side of the block, it’s passing into a less dense medium so it speeds up and bends away from the normal (some of the light is also reflected at this boundary)
5. The light ray that emerges on the other side of the block is now travelling in the same direction it was to begin with - it’s been refracted towards the normal and then back again by the same amount
6. You can measure the angles between the rays and the normal to work out the refractive index of the material in the block

Question 19

When light slows down, it bends towards/away from the Normal

When light speeds up, it bends towards/away from the Normal

Answer 19

When light slows down, it bends towards the Normal

When light speeds up, it bends away from the Normal

Question 20

What happens when light travels through a block?

Answer 20

As the light passes from the air into the block (a denser medium), it bends towards the normal. This is because it slows down

When the light reaches the boundary on the other side of the block, it’s passing into a less dense medium so it speeds up and bends away from the normal

The light ray that emerges on the other side of the block is now travelling in the same direction it was to begin with - it’s been refracted towards the normal and then back again by the same amount

Question 21

What is the refractive index and its equation?

Answer 21

the refractive index of a material (n) is the ration of the speed of light in free space (air) to the speed ofl ight in the material:

n = speed of light in a vaccum (c) ÷ speed on light in material (v)

n = c ÷ v

Question 22

Light slows down/speeds up in glass, so the refractive index of glass is high/low

The refractive index of water is a bit higher/lower so the light doesn’t slow down/speed up as much in water as in glass

Answer 22

Light slows down a lot in glass, so the refractive index of glass is high

The refractive index of water is a bit lower so the light doesn’t slow down as much in water as in glass

Question 23

What is Snell’s Law?

Answer 23

when an incident ray passes into a material: n = sin i ÷ sin r

Question 24

A beam of light travels from air into water. The angle of incidence is 23^o. Refractive index of water = 1.33. Calculate the angle of refraction to the nearest degree

Answer 24

n = sin i ÷ sin r

sin r = sin i ÷ n

sin 23^o ÷ 1.33

=0.29…

r = sin^-1 (0.29…)

= 17^o

Question 25

How do you find the refractive index of glass using a glass block?

Answer 25

1. Draw around a rectangular glass block on a piece of paper and direct a ray of light through it at an angle. Trace the incident and emergent rays, remove the block, then draw in the refracted ray between them
2. You then need to draw in the normal at 90^o to the edge of the block, at the point where the ray enters the block (this is where you measure the ray of light)
3. Use a protactor to measure the angle of incidence (i) and the angle of refraction (r)
4. Calculate the refractive index (n) using Snell’s Law: n = sin i ÷ sin r

N.B. The refractive index should be around 1.5

Question 26

What happens to the angle of refraction as you increase the angle of incidence?

Answer 26

as the angle of incidence increases, the angle of refraction gets close to 90^o

Question 27

Desrcibe an experiment to demostrate total internal reflection using a semicircular block

Answer 27

1. Draw around a semicircular glass block on a piece of paper and aim an incident light ray at the curved edge of the block so that it always enters at right angles to the edge. Trace the incident and emergent rays, remove the block, then draw in the refracted ray between them
2. You then need to draw in the normal at 90^o to the edge of the block, at the point where the ray enters the block (this is where you measure the ray of light)
3. Use a protactor to measure the angle of incidence (i) and the angle of refraction (r)
4. Calculate the refractive index (n) using Snell’s Law: n = sin i ÷ sin r

Question 28

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

Answer 28

most of the light passes out but a little bit of it is internally reflected

Question 29

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

Answer 29

the emerging ray comes out along the surface

there is quite a bit of internal reflection

Question 30

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

Answer 30

no light comes out

it’s all internally reflected i.e. total internal reflection

Question 31

What is total internal reflection?

Answer 31

complete reflection of a light ray reaching a boundary at an angle greater than the critical angle

Question 32

See Experiment Write Ups for the Snell’s Law Experiment

Question 33

What does the angle of incidence equal when the angle of refraction = 90^o

Answer 33

when the angle of refraction = 90^o the angle of incidence = the critical angle

Question 34

What is the equation for finding the critical angle?

Answer 34

sin C = 1 ÷ n (the refractive index of the material)

Question 35

The higher the refractive index, the higher/lower the critical angle

Answer 35

The higher the refractive index, the lower the critical angle

Question 36

The critical angle is 42^o. Calculate the refractive index

Answer 36

sin C = 1 ÷ n

n = 1 ÷ sin C

= 1 ÷ (sin C)

= 1 ÷ 0.66…

= 1.49 (2 d.p)

Question 37

How do optic fibres use total internal reflection?

Answer 37

Information, in the form of pulses of light, is sent down bundles of optical fibres

optic fibres made of plastic or glass consist of a central core surrounded by cladding that has a lower refractive index

the core of the fibre is so narrow that light signals passing through it always hit the core-cladding boundary at angles higher than the critical angle - so the light is always totally internally reflected

it only stops working if the fibre is bent too sharply

Question 38

How do you use total internal reflection in a periscope?

Answer 38

Two right angle prisms can be used to make a periscope.
At the back face of the prisms there is total internal reflection.
Please see the picture below.

Question 39

Different wavelengths of light refract by the same/different amounts.

What does this do to white light as it enters a prism?

Answer 39

Different wavelengths of light refract by different amounts

White light disperses into different colours as it enters a prism

Question 40

What is the relationship between wavelength and refraction?

Answer 40

The shorter the wavelength of the light, the more it is refracted

Question 41

Which colour is refracted the most? Why?

Answer 41

violet light is refracted the most because it has the shortest wavelength

Question 42

Which colour is refracted the least? Why?

Answer 42

red light is refracted the least as it has the longest wavelength