P14: Light

Question 1

What happens when a wave hits a boundary with a different material?

Answer 1

It is either reflected, absorbed or transmitted.
(in some cases, waves can change direction when passing through the material (refraction))
(if it is absorbed, the energy of the wave is absorbed by the material)
Which of these happens depends on the material and wavelength of the wave.

Question 2

How can ray diagram be used to work out the position of an object in a mirror?

Answer 2

Draw an incident ray from the object to the mirror, then draw the normal on the mirror, and the reflected ray (angle of incidence = angle of reflection). Also draw arrows showing the direction of both reflected rays.
Extend the reflected rays towards the mirror - where the lines meet is the location of the reflected image.

Question 3

What is refraction?

Answer 3

When waves change direction when they change speed when moving from one medium to another.
e.g. when light enters a glass block, its velocity decreases, which will change the direction of the waves.
The object will have appeared to have shifted position

Question 4

What happens to waves when they slow down?

Answer 4

Bend towards the normal

Question 5

What happens to waves when they speed up?

Answer 5

Bend away from the normal

Question 6

What is the exception to refraction?

Answer 6

If light from the object is coming perpendicular (along the normal) to the glass block, no refraction takes place.

Question 7

What is a convex lens?

Answer 7

The one that bulges outward - looks like an egg.
(thicker at the centre than at the edges)

Question 8

What is the symbol for a convex lens?

Answer 8

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Question 9

What is the key feature of all lenses?

Answer 9

They refract light.
For convex lenses, light bends towards the normal upon entering, then away from the normal upon exiting.

Question 10

Why is the central ray passing into a convex lens not refracted?

Answer 10

It passes along the normal - the principle axis.

Question 11

What is the principal focus?

Answer 11

When the rays refract, they become focused on a point, called the principal focus (F).

Question 12

What is the focal length?

Answer 12

Distance between centre of the lens and the principal focus.

Question 13

What affects focal length?

Answer 13

Different lenses have different focal lengths depending on the strength of the lens

Question 14

How would you draw a ray diagram for a convex lens?

Answer 14

Draw a line between the top of the object and the origin of the graph.
Draw another line from the top of the object that is parallel to the principal axis. When it hits the lens its refracted through the principal focus.
Where the 2 lines meet shows the top of the image.
The image will have diminished, be real and be inverted.

Question 15

What is a real image?

Answer 15

An image that can be seen on a screen.
Happens when rays meet at a point.

Question 16

What is a virtual image?

Answer 16

An image that cannot be seen on a screen.
The rays no not meet at a point.

Question 17

What effect does moving an object closer to a convex lens have?

Answer 17

The object is still real and inverted, but it is magnified.

Question 18

What happens if an object is more than 2 focal lengths from the convex lens?

Answer 18

It is diminished, inverted and real.

Question 19

What happens if an object is between 1 and 2 focal lengths from the lens?

Answer 19

The object is magnified, inverted and real.

Question 20

How would you draw the ray diagram for an object that is less than 1 focal length from a lens?

Answer 20

Draw ray from top to origin.
Draw ray parallel to the principal axis that passes through the principal focus.
You will find that they do not meet, therefore extend them in the opposite direction. Where they meet shows the position of the image

Question 21

What happens if an object is less than 1 focal length from the lens?

Answer 21

It is virtual, upright and magnified.

Question 22

What is the only time when a convex lens produces a virtual image?

Answer 22

When it is used as a magnifying glass.

Question 23

What is a concave lens?

Answer 23

A lens which is thinner in the centre than at the edges
(caves in)

Question 24

What is the symbol for a concave lens?

Answer 24

↓
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↑

Question 25

What do concave lenses do to light rays?

Answer 25

Make them spread out - light rays diverge.

Question 26

Where is the principal focus on a concave lens?

Answer 26

When you extend the diverging light rays, they will meet at a principal focus (F). They only appear to be coming from the principal focus, but they don’t actually come from there.

Question 27

Draw a ray diagram for an object seen through a concave lens

Answer 27

Draw line from top to origin.
Draw line parallel to the principal axis, then refract away from principal axis.
Extend the second ray, as it must appear to have come from the principal focus. Where the lines meet shows the position of the image.
The image is diminished, virtual and upright.

Question 28

What is specular reflection?

Answer 28

All light rays are reflected in a single reflection - takes place on smooth surfaces and produces an image.

Question 29

What is diffuse reflection?

Answer 29

When light rays are reflected they are scattered - takes place on rough surfaces and does not produce an image.

Question 30

How do coloured filters work?

Answer 30

They absorb specific wavelengths and transmit other wavelengths - e.g. a red filter only transmits red light but absorbs all other wavelengths.

Question 31

What is translucent?

Answer 31

An object that lets all light through, but scatters it

Question 32

What is transparent?

Answer 32

An object which lets all light through

Question 33

What is opaque?

Answer 33

An object which does not transmit any light