Chapter 14 - Light

Question 1

Plane mirror

Answer 1

Perfectly flat surface

Question 2

Reflection

Answer 2

• light consists of waves, when plane waves reflect from a flat barrier, the reflected waves are at the same angle to
  the barrier as the incident waves
• when each point on the wavefront reaches a barrier, it creates a wavelet moving away from the barrier
• this wavelet lines up with the previous reflected wavelets to form a reflected wavefront moving away from the barrier
• all parts of a wavefront move at the same speed
• means that the reflected wavefront is at the same angle to the barrier as the incident wavefront

Question 3

Plane waves - Their reflected and incident waves

Answer 3

The reflected waves and the indicent waves have the same frequency and they travel at the same speeds they have the same wavelength.

Question 4

Law of reflection

Answer 4

Measurements show that for any light ray reflected by a plane mirror, the angle of reflection = angle of incidence

Question 5

Features of a reflection

Answer 5

• the line perpendicular to the mirror is called the normal
• the angle of incidence is the angle between the incident ray and the normal
• the angle of reflection is the angle between the reflected ray and the normal

Question 6

Real and virtual images

Answer 6

• images formed by a plane mirror is virtual, upright (the same way up as the object) and laterally inverted (back to front but not upside down)
• a virtual image is formed at a place where light rays appear to come from after they have been reflected (or refracted)
• virtual images can’t be projected onto a screen like the movie images at a cinema - an image that can be seen on a screen is described as a
  real image because it is formed by focusing light rays onto the screen

Question 7

Specular reflection

Answer 7

• mirror has a smooth surface that reflects light rays without scattering them, this is why you can see a clear image when you look in a mirror
• reflection from a smooth surface is called specular reflection because parallel light rays are reflected in a single direction

Question 8

Diffuse reflection

Answer 8

• parallel light rays reflected from a rough surface are scattered in different directions
• if you polish a rough surface like a dusty table top to make it smooth, you might see a reflection in the surface
• reflection from a rough surface is called diffuse reflection because the light is scattered in different directions

Question 9

Why do ambulances carry a mirror image sign on the front?

Answer 9

So the driver of the vehicle infront can read the sign when they look in their car mirror, laterally inverted.

Question 10

What do lenses do?

Answer 10

Changes the direction of light passing through it - this change of direction is called refraction

Question 11

Refraction

Answer 11

• a property of all kinds of waves, including light and sound
• refraction happens to water waves when they cross a boundary between deep and shallow water at a non-zero angle to the boundary
• change of speed at the boundary causes them to change direction

Question 12

Refraction - simple definition

Answer 12

When a wave crosses a boundary between materials at an angle, it change direction.

Question 13

Refraction of light

Answer 13

• light waves are refracted when they travel across a boundary between air and a transparent medium or between two transparent media
• this is because the speed of light changes at this kind of boundary

Question 14

Describe the refraction of light

Answer 14

• the change of direction of each ray relative to the normal at each boundary is towards the normal when light travels from air into glass
• the change of direction of each ray relative to the normal at each boundary is away from the normal when light travels from glass to air

Question 15

Density and the direction of refraction

Answer 15

• change of direction happens because light travels more slowly in glass than in air
• because light travels more slowly in glass than in air, glass is said to be ‘optically more dense’ than air
• when light enters a more-dense medium, it is refracted towards the normal
• when light enters a less dense medium, it is refracted away from the normal

Question 16

What else can happen when waves cross a boundary?

Answer 16

• partial reflection can also happen as well as refraction
• this is why you might see a faint mirror image of yourself looking at a window
• the waves that cross the boundary lose energy at the boundary and so have a smaller amplitude than that of the incident waves

Question 17

What happens when light waves move from air into glass?

Answer 17

1. when the wavefronts start to move into the glass, those parts of the wavefronts slow down
2. this causes those parts of the wavefronts to get closer together, which results in their wavelength getting smaller
3. this ultimately leads to the waves changing direction towards the normal - in other words, the waves are refracting

Question 18

What is a wavefront?

Answer 18

• imagine a few identical waves travelling together, one on top of the other
• draw a line through all the waves’ peaks, or draw a line through all the waves’ troughs
• basically, the wavefront is an imaginary line that connects all the same points in a set of waves
• you don’t do the normal to the wavefronts, you do it to the direction in which the wave is travelling

Question 19

Refraction rules

Answer 19

• a light ray changes direction towards the normal when it travels from air into glass. the angle of refraction r is smaller than the angle of incidence i
• a light ray changes direction away from the normal when it travels from glass into air. the angle of refraction r is greater than the angle of incidence

Question 20

Wavefronts moving along the normal

Answer 20

The waves slow down as they pass into the glass.

In this case, the whole wavefront slows down at the same time which means that the waves don’t change direction.

Question 21

Investigation refraction (core practice)

Answer 21

• boundaries between different substances refract light by different amounts.
  1. place a transparent rectangular block on a piece of paper and trace around it. use a ray box or a laser to shine a ray of light in the middle of one side of the block
  2. trace the incident ray and mark where the light ray emerges on the other side of the block. remove the block and with a straight line, join up the incident ray and the emerging point to show the path of the refracted ray through the block
  3. draw the normal at the point where the light ray entered the block. use a protractor to measure the angle between the incident ray and the normal (the angle of incidence, I) and the angle between the refracted ray and the normal
  (the angle of refraction, R)
  4. repeat this experiment using rectangular blocks made from different materials, keeping the incident angle the same throughout
• angle of refraction changes for different materials, the difference is due to their different optical densities
• do this in a dim room so you can clearly see the light rays, you can either use a ray box or a laser to produce thin rays of light so you can easily see the middle of the ray when tracing it and measuring angles from it

Question 22

Wavelength of light

Answer 22

Increases from violet to red across the visible spectrum.