Unit 5: Ray Model of Light

Question 1

What are objects such as the sun, lamps and fire called and how do we see them?

Answer 1

• Objects such as the sun, lamps and fire produce light.
- They are called luminous objects and are sources of light.

• We see luminous objects because the light they produce enters our eyes directly.

Question 2

What are objects such as the moon or a book called and how do we see them?

Answer 2

• Most objects, such as the moon or a book, do not produce light.
- They are called non-luminous objects and are not sources of light.

• We see the book because light from the sun or a lamp bounces off (or reflects) the book and enters our eyes.

Question 3

How does light travel?

Answer 3

• Projector lamps and spotlights produce beams of light.
• Beams are represented in diagrams by a bunch of rays.
• Light travels from the light source to the object then to our eyes.

Question 4

What are the 4 types of light rays?

Answer 4

1. 1 ray of light
2. parallel beams
3. convergent beams (going upwards)
4. divergent beams (going downwards)

Question 5

What happens when light is blocked?

Answer 5

• Shadows are formed when some rays of light are blocked by an object while the other rays continue to travel in straight lines.

• The area behind the object that receives no light is the shadow.

Question 6

How is light reflected?

Answer 6

• A property of light allows us to see ourselves in the mirror. When we look into a mirror, what we see is called an image.

• The image in a mirror is formed when light rays bounce off the mirror and travel into our eyes.

• This bouncing of light off a mirror is called reflection.

Question 7

What are the laws of reflection? (2)

Answer 7

• The incident ray, the reflected ray and the normal to the reflecting surface all lie on the same plane.

• The angle of incidence (i) is equal to the angle of reflection (r). (i = r)

• When a ray of light reflects off a smooth surface, the angle of incidence is equal to the angle of reflection.

• We are able to observe this effect of light when we view images in the mirror. As you look at the image in the mirror, light travels from the object, reflects off the mirror and then travels into your eyes.

Question 8

Characteristics of an image formed by a plane mirror (5)

Answer 8

• The image formed by a plane mirror is at the same distance from the mirror as the object from the mirror.
• The image is upright.
• The image is the same size as the object.
• The image is a virtual image. It cannot be formed on a screen.
• The image is laterally inverted as the mirror turns the image sideways.

Question 9

What is the meaning of Incident ray?

Answer 9

The light ray that hits the reflecting surface (i.e. plane mirror).

Question 10

What is the meaning of Point of incidence (O)?

Answer 10

The point at which the incident ray hits the reflecting surface.

Question 11

What is the meaning of normal (scientific)?

Answer 11

The imaginary line that is perpendicular to the reflecting surface at the point of incidence.

Question 12

What is the meaning of reflected ray?

Answer 12

The light ray that bounces off the reflecting surface.

Question 13

What is the meaning of Angle of incidence (i)?

Answer 13

The angle between the incident ray and the normal.

Question 14

What is the meaning of Angle of reflection (r)?

Answer 14

The angle between the reflected ray and the normal.

Question 15

Is the angle of incidence i, equal to the angle of reflection, r?

Answer 15

Yes
i = r

Question 16

Constructing ray diagrams (methods)

Answer 16

Method 1 (preferred)
1. Draw the incident ray
2. Draw the reflected ray using Law of Reflection
3. Extend the reflected rays behind the mirror to locate the image

Method 2
1. Draw the location of the image
2. Draw the reflected rays from the image to the eye
3. Draw the incident rays from the points of incidence

Question 17

Constructing ray diagrams (important things)

Answer 17

• If the mirror is tilted, the obj and image will also be tilted (make sure its 90° to the mirror)
• The normal line must be dotted
• Draw dotted circle for the virtual image because it is virtual and cannot be seen
• Anything that cannot be seen must be drawn with dotted lines.

• Dotted lines are used for the light rays behind the mirror. This is because these rays are “virtual”. They do not really exist.

• Solid lines with arrowheads are used for the light rays reflected off the mirror into the eye.

Question 18

Steps to draw a ray diagram? (3)

Answer 18

Step 1: Locate the position of the image behind the mirror
Since distance of the object from the mirror = distance of the image from the mirror, we can
locate the position of the image behind the mirror.

Step 2: Draw the reflected ray from the image to the eye
i. Dotted lines are used for the light rays behind the mirror. This is because these rays are “virtual”. They do not really exist.
ii. Solid lines with arrowheads are used for the light rays reflected off the mirror into the eye.

Step 3: Draw the incident rays
Draw the incident rays by drawing lines joining the object to the points of incidence on the mirror
surface.
The arrowheads on these lines should point towards the mirror.

Question 19

Properties of a concave mirror (4)

Answer 19

• Upright (very near)
• Magnified
• Inverted (far)
• Concave mirrors form upright magnified images if the objects are close to the mirrors.

• This property makes them useful as a dentist’s mirror, and in a microscope.

image formed on mirror is bigger than actual object but range of image is small

Question 20

Properties of a convex mirror (4)

Answer 20

• Upright
• Smaller than object (diminished)
• Large field of view
• Convex mirrors form upright images that are always smaller than the objects. They also have a large ‘field of view’, that is, they allow many things to be seen.

• This property makes them useful as security mirrors in shops, and as mirrors at blind corners of roads.

image formed on mirror is smaller than actual object but range of view is bigger

Question 21

Regular reflection

Answer 21

• When parallel rays of light hit a smooth surface, they are reflected as parallel rays.
• This is called regular reflection. In regular reflection, the image is clear and undistorted.

Question 22

Types of Reflections (2)

Answer 22

There are two kinds of reflections, depending on whether the surface is smooth or rough. They are Regular reflection and Diffuse reflection.

Question 23

Diffuse reflection

Answer 23

• Rough surfaces, such as frosted glass, have many different angles of incidence. Hence, light rays are reflected off the surface at different angles.
• This is called diffuse reflection. In diffuse reflection, the image is distorted.

Question 24

Uses of plane mirrors

Answer 24

• The rearview mirror inside a car helps the driver to see traffic behind the car.
• A plane mirror on a wall makes a room look bigger.
• A simple periscope makes use of mirrors. It can be used in buses to enable the driver to see people on the upper deck.

Question 25

Properties of Light? (2)

Answer 25

• Light travels at about 3.00 x 10⁸ m/s in a vacuum. (faster than sound!)
• Light travels in a straight line.

Question 26

How does a periscope work?

Answer 26

• A periscope is an instrument used to observe from a concealed position.
• A periscope can be used to look over high obstacles (e.g. a wall).
- They are also used in submarines to have a view of the water surface.
• Light enters one end of the device and makes a 90° turn when it reflects off a plane mirror set at 45°. The second mirror, also set at 45°, directs the light through another 90° turn so that it travels towards the observer.
• Since light rays from the object are reflected by two mirrors, a lateral inversion caused by the first mirror is corrected by the second mirror. This makes the image appear without being laterally inverted.

Question 27

Definition of Refraction of Light?

Answer 27

• Refraction is the bending of light at the boundary between two transparent mediums (of different optical densities) due to a change in speed of light as it passes from one material to another.

Question 28

Refraction of Light

Answer 28

• Light changes direction when it travels from one transparent material (medium) to another transparent material of a different optical density (e.g. from air to glass and vice versa).
• When light passes from one transparent medium to another, it may slow down or travel faster.
• This change in the direction of light is known as refraction.
- For example, this straw appears to be broken because of refraction.

• Refraction is the bending of light at the boundary between two transparent mediums (of different optical densities) due to a change in speed of light as it passes from one material to another.
• Light travels slower in materials that are optically denser.
• This change in the speed of light causes the light to bend or change its direction.

Question 29

What happens when a light ray travels from an optically less dense medium to an optically denser medium?

Answer 29

• The speed of light decreases when a light ray travels from an optically less dense medium to an optically denser medium, thus it bends towards the normal.

less dense medium → denser medium = travels slower and bends towards normal

e.g.
When a light ray travels from air to glass, it bends towards the normal. The speed of light decreases when it travels from a less dense medium (air) to a denser medium (glass). So, the light ray bends towards the normal.

Question 30

What happens when a light ray travels from a denser medium to a less dense medium?

Answer 30

• The speed of light increases when it travels from a denser medium to a less dense medium. Thus, the light ray is refracted away and bends away from the normal.

denser medium → less dense medium = travels faster and bends away from normal

e.g.
When a light ray travels from glass to air, it is refracted away from the normal. The speed of light increases when it travels from a denser medium (glass) to a less dense medium (air). So, the light ray bends away from the normal.

Question 31

Since light is travelling from an optically less dense to an optically denser medium, it …

Answer 31

Since light is travelling from an optically less dense to an optically denser medium, it travels slower and bends towards the normal.

Question 32

Since the incident ray is perpendicular to the surface of the glass block, it …

Answer 32

Since the incident ray is perpendicular to the surface of the glass block, it will not bend. (angle of incidence = 0°)

Question 33

Since light is travelling from an optically denser to an optically less dense medium, it …

Answer 33

Since light is travelling from an optically denser to an optically less dense medium, it speeds up and bends away from the normal.

Question 34

Effects of Refraction ? (1)

Answer 34

• Refraction makes objects under glass or water appear higher than they really are. We say that the apparent depth is less than the real depth.
• Refraction also causes a swimming pool to appear shallower.

Question 35

What colours are called the spectrum of white light?

Answer 35

• These colours are called the spectrum of white light. The spectrum consists of seven colours: Red, Orange, Yellow, Green, Blue, Indigo and Violet (ROYGBIV).

Question 36

What is dispersion?

Answer 36

• The separation or splitting of white light into its separate colours is called dispersion.
- The same dispersion process happens in the sky. Rainbows are often seen after rain. Water droplets in the air act like small prisms and help to disperse white light into the seven colours of the rainbow.

Question 37

What is the separation or splitting of white light into its separate colours called?

Answer 37

Dispersion (of white light)

Question 38

In the dispersion of white light, which colour deviates the most and which colour deviates the least?

Answer 38

• Different colours of light have different frequencies and wavelengths but travel at a speed of 3 x 10⁸ m/s.
• The colour that deviates (bends) the least by the prism is red while the colour that deviates (bends) the most by the prism is violet.

• As the refracted colours of light leave the prism, they are bent again, but not in their original path. This is because the face of the prism through which the light leaves is not parallel to the face through which it enters.

Question 39

How do we prove that white light is a mixture of different colours (or to recombine the colours of the spectrum)? (2 ways)

Answer 39

• To prove that white light is a mixture of different colours (or to recombine the colours of the spectrum), we can either place another prism that is inverted to the first prism or by using the Newton’s disc.

• A second prism can be used to recombine the spectrum to give white light.

• A colour wheel (also known as Newton’s disc) can be turned quickly to mix the colours, making the wheel look white.

Question 40

Why does a white ray of light leave a prism with 7 different colours?

Answer 40

• As the refracted colours of light leave the prism, they are bent again, but not in their original path. This is because the face of the prism through which the light leaves is not parallel to the face through which it enters.

Question 41

How is the normal determined?

Answer 41

The normal must be perpendicular to the surface (90 degrees)