Light10

Question 1

What’s the law of reflection?

Answer 1

The angle of incidence = the angle of reflection

Question 2

What happens when light reflects from an uneven surface?

Answer 2

The light reflects at different angles so you get a diffuse reflection

Question 3

What happens when light reflects from an even, smooth, shiny surface?

Answer 3

It’s all reflected at the same angle and you get a clear reflection.

Question 4

What’s the normal line?

Answer 4

It’s an imaginary line that’s perpendicular to the surface at point of incidence(the normal is shown as a dotted line)

Question 5

What’s the angle of incidence?

Answer 5

It’s the angle between the incoming wave and the normal.

Question 6

What’s the angle of reflection?

Answer 6

The angle between the reflected ray and the normal.

Question 7

What are virtual images?

Answer 7

Virtual images are formed when light rays bouncing of the mirror are diverging so the object appears to come from behind the mirror.

Question 8

Why do waves travel at different speeds in different substances?

Answer 8

Different substance have different densities. Light waves generally travel slower in denser substances.

Question 9

What happens to a wave when it crosses into another substance?

Answer 9

It changes speed

Question 10

What happens when a wave hits another substance at 90°?

Answer 10

The wave slows down but carries on in the same direction.

Question 11

What happens if a wave enters a different medium at a non perpendicular angle?

Answer 11

One part of the wave that hits the denser medium first slows down, but the other part carries on at first at the faster speed so the wave changes direction. IT HAS BEEN REFRACTED.

Question 12

What’s a ray diagram?

Answer 12

A ray diagram shows the path that a wave travels.

Question 13

How do you draw a ray diagram?

Answer 13

Draw the normal, then draw the incident ray which meets the normal, the draw the refracted ray(if the object is denser the angle should be smaller if the object is less dense the angle should be larger)

Question 14

How do you draw a plane mirror?

Answer 14

From the object you draw two rays when the meet the normal you treat the like normal incident and reflected rays NORMAL LINES, then behind the mirror you repeat the two incident rays just on the opposite side of the mirror BUT USE DASHED LINES

Question 15

Describe a practical for rays passing through a glass block when it’s refracted twice:

Answer 15

• use a rectangular block of a particular material resting on a piece of paper
• shine a light ray at an angle into the block, some light is reflected but most passes through the glass and gets refracted as it does so
• you trace the incident and emergence ray and draw in the refracted ray by joining the two line
• then label the incident ray, angle; refracted ray, angle and emergence ray, angle

Question 16

In a glass block light experiment which two angles are equal?

Answer 16

Angle of incidence and angle of emergence

Question 17

What do triangular prisms do?

Answer 17

They disperse light

Question 18

Why do triangular prisms disperse light?

Answer 18

Different wavelengths of light refract by different amounts so white light(which is a mix of all visible frequencies) disperses into different colours as it enters a prism and the different wavelengths are refracted by different amounts. A similar effect happens when the light leaves the prism which means you get a rainbow.

Question 19

What is the visible light spectrum? And what refracts the least and most?

Answer 19

Red Orange York Green Blue Indigo Violet
(Richard of York gave battle in vein)
Red refracts the least and violet refracts the most

Question 20

What does the refractive index of a material tell you?

Answer 20

How fast light travels in that medium

Question 21

What’s the refractive index of a material is defined as?

Answer 21

Speed of light in a vacuum(c)
Refractive index(n) =——————————-
Speed of light in that material(v)

Question 22

What’s Snell’s law?

Answer 22

Sin i
N = ——-
Sin r

Question 23

Describe a practical to find the refractive index of glass using a glass block:

Answer 23

• draw round a rectangular glass block and direct a light through it at different angles such as 20,30,40,50,60 working out the angle of refraction for all of these
• then plot a graph of sin i against sin r and the gradient is the refractive index

Question 24

Describe a practical which uses a semicircular block to show total internal reflection:

Answer 24

• draw round the glass block, aim the incident ray at the glass block so that the light enters at 90° this means it doesn’t bend when it enters the block only when it leaves the block
• as you increase the incident angle the refraction angle gets closer to 90°
• eventually i reacher 90° this is known as the critical angle
• record your results in a table

Question 25

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

Answer 25

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

Question 26

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

Answer 26

The emerging ray comes out along the surface. There is more total internal reflection. The refracted angle is 90°

Question 27

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

Answer 27

No light comes out. It’s all internally reflected, TOTAL INTERNAL REFLECTION

Question 28

What the equation to work out the critical angle?

Answer 28

1
Sin C = ——
n

C=critical angle
N=refractive index

Question 29

How do optical fibres use total internal reflection?

Answer 29

• Optical fibres are made of plastic or glass and consist of a central core surrounded with a lower refractive index
• the core of the fibre is so narrow that light signals passing through always hit the core cladding at an angle higher than C so the light is TIR
• it only stops working if the fibre is bent to sharply

Question 30

How do prisms use total internal reflection?

Answer 30

• total internal reflection allows us to see objects that aren’t directly in our line of sight. This is how periscopes work
• the ray of light travels into a prism where it’s TIR by 90°
• it then travels to another prism lower down and is TIR by another 90°
• the ray is now travelling parallel to its initial path but at a different height

Question 31

What type of wave is visible light?

Answer 31

Visible light is a transverse wave

Question 32

What does a ray diagram show?

Answer 32

A ray diagram shows the path that a wave travels along. Rays are always drawn as straight lines.