Light

Question 0

What is specular reflection?

Answer 0

All the incident ray is reflected at the same angle

Question 1

What is a plane mirror?

Answer 1

A flat mirror

Question 3

What is diffuse reflection?

Answer 3

All the incident ray is reflected at lots of different angles

Question 4

What is the angle of incidence equal to?

Answer 4

The angle of reflection

Question 5

Give four characteristics of a reflected image

Answer 5

Objects are the same size
Same distance away from the mirror
The image is virtual
Laterally inverted - left and right sides are swapped.

Question 6

What is refraction?

Answer 6

The bending (changing direction) of a light ray as it travels from one transparent substance to another one with different optical density

Question 7

What happens when a ray of light enters an optically denser substance?

Answer 7

It bends towards the normal

Question 8

What happens when a ray of light leaves an optically denser substance?

Answer 8

It bends away from the normal

Question 9

Will a refracted ray ever bend beyond the normal?

Answer 9

No (so there is no bending when a ray crosses a boundary at a right angle)

Question 9

What is the relationship between sin(i) sin(r) and the refractive index(n)

Answer 9

Sin(i) / sin(r) = n

Question 10

If the two boundaries of the substance are parallel what will the rays be?

Answer 10

Parallel

Question 11

What is Snell’s Law?

Answer 11

When an incident ray passes into a material: n = sin(i) / sin(r)
When you divide sin(i) by sin(r) you get the same number (for a particular transparent substance)

Question 12

What is Snell’s Law?

Answer 12

When an incident ray passes into a material: n = sin(i) / sin(r)
When you divide sin(i) by sin(r) you get the same number (for a particular transparent substance) - the ratio is constant

Question 13

Describe what happens to the light beam inside the prism (right angled triangle entering on the left)

Answer 13

It is totally internally reflected off the longest face (angle of incidence >or equal to>critical angle)

Question 14

What happens when a ray of white light passes through a prism?

Answer 14

The white light emerges as a band of colours called a spectrum. This is caused because light is a mixture of colours and each colour travels through the prism at a slightly different speed, so each colour is refracted at a different angle. The prism has a different refractive index for each colour. This process is called dispersion.

Question 15

Describe what happens to the light beam inside the prism (right angled triangle entering at the base)

Answer 15

It is totally internally reflected off the two short faces (angle of incidence>or equal to>critical angle

Question 16

What is total internal reflection used in?

Answer 16

Prisms in periscopes, binoculars and bicycle reflectors, optic fibres

Question 17

What is a virtual image?

Answer 17

A virtual image is when the rays are diverging so the light from the object APPEARS to be coming from a completely different place. E.g. A mirror

Question 18

When does total internal reflection happen?

Answer 18

When you go from a more optically dense object to a less optically dense object and when the angle of incidence is greater than the critical angle

Question 19

Is looking through a magnifying lens real or virtual?

Answer 19

Virtual because the image looks bigger than it actually is.

Question 20

3 things you need to say to describe an image properly:

Answer 20

1. How big it is compared to the object.
2. Whether it’s upright or inverted (upside down) relative to the object.
3. Whether it’s real or virtual.

Question 21

What is a luminous object?

Answer 21

Objects that emit their own light.

Question 22

What are non-luminous objects?

Answer 22

Objects that just reflect light, this is the case for most objects.

Question 24

How do optical fibres actually work?

Answer 24

Optical fibres are very thin strands made from two types of glass. The optically less dense glass surrounds an optically more dense glass. As the fibres are very narrow, light always strikes the boundary of the two glasses at an angle greater than the critical angle. All the light is reflected so no light escapes across the boundary, creating a ‘light pipe’. providing a path the light follows even when the fibre is curved

Question 25

What is meant by the term total internal reflection?

Answer 25

At an angle of incidence greater than the critical angle, all light is reflected back, and there is no more refraction. The reflection occurs inside the more optically dense medium

Question 26

What is the critical angle (c) ?

Answer 26

The angle of incidence at which total internal reflection begins

Question 27

What is the refractive index of a material?

Answer 27

The ratio of the sine of the angle of refraction and the sine of the angle of incidence.
It tells you how fast light is travelling in a material

Question 28

Describe an experiment to find the refractive index of glass

Answer 28

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 need to draw in the normal at 90° to the edge of the block, at the point where the ray enters the lock
3) Use a protractor to measure the angle of incidence (i) and the angle of refraction (r). Calculate the refractive index. Repeat to make the experiment reliable.

Question 29

What is the speed of light?

Answer 29

300,000,000 m/s

Question 30

How does the prismatic periscope work?

Answer 30

Light passes normally through the surface AB of the first prism (it enters the prism at 90°) and so is undeviated.
It then strikes the surface AC (hypotenuse) of the triangular prism at an angle of 45°.
The critical angle for glass is 42° so the ray is totally internally reflected and turned through 90°.
On emerging from the first prism the light travels to a second prism which is positioned such that the ray is again totally internally reflected.
The ray emerges parallel to the direction in which it was originally travelling.

Question 31

What are the advantages of sending signals using optic fibres?

Answer 31

They are less prone to noise/heating

They can send more information/data per second