P14: Light (Y11-Spring 1)

Question 1

🟢 What is the Law of Reflection

Answer 1

• The line perpendicular to the mirror is called the mirror
• 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

Measurements show that for any light ray reflected by a plane mirror:

The angle of incidence = the angle of reflection

Question 2

🟢 What are real and virtual images

Answer 2

The image formed by a plane mirror is vortual, upright (the same way up as the object), and laterally inverted (back to front but no upside down).

A virtual image is formed at a place where light rays appear to come onto a screen after they have been reflected (or refracted). It can’t be projected onto a screen like the movie images you see at the cinema.

An image that can be seen on a screen is descrived as a real image because it is formed by focusing light rays onto the screen.

Question 3

🟢 What is Specular Reflection

Answer 3

Reflection from a smooth surface is called ‘specular reflection’ as the parallel light rays are reflected in a single direction parallel to each other still.

Question 4

🟢 What is Diffuse Reflection

Answer 4

Light rays reflected off a rough surface in different directions is called ‘diffuse reflection’

Question 5

🟢 What happens to the property of the wave as it travels from deep to shallow water

• Velocity
• Wavelength
• Frequency
• Direction of the Wave

Answer 5

Velocity:
Decreases

Wavelength:
Decreases

Frequency:
Stays the same

Direction of the Wave:
Benss towards the normal

Question 6

🟢 What are the Two Rules of Refraction (when light moves from a less dense medium to a dense medium and when light moves from a more dense medium to a less dense medium)

Answer 6

Wave change speed when they pass between substances of different densities, this cause them to change direction – this is ‘refraction’

If light rays move from a less dense medium (air) to a more dense medium (water/glass) they
move towards the normal.

If light rays move from a more dense medium (water/glass) to a less dense medium (air) they ‘move’ away from the normal.

Question 7

🟢 What happens when lights enters and then exits a glass block

Answer 7

When light enters the glass block it slows down, this is because glass is more dense than air.

The wavelength of the light ray decreases and the direction moves towards the normal line

As the ray moves out of the glass block into the air it speeds up, this is because air is less dense than glass.

The wavelength of the light ray increases and the direction moves away from the normal line.

Question 8

🟢 When does Partial Reflection happen?

Answer 8

When waves cross a boundary between two materials, partial reflection can also happen as well as refraction. This is why you might see a faint mirror image of yourself when you look at a window. The waves that cross that boundary lose energy at the boundary and so have a smaller amplitude that turn of the incident waves.

Question 9

🟢 What are the 2 rules of Refraction

Answer 9

The investigation should show that a light ray:

• Changes direction Towards the normal when it travels into glass. The angle of refraction, r, is smaller than the angle of incidence, i.
• 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, i.

Question 10

🟢 What does the colour of the surface of an opaque object depend on

Answer 10

The colour of the surface of an opaque depends on chemicals called pigments in the surafacs in the materials. Colour also depends on the range of wavelengths in the incident light. Pigments absorb light of specific wavelengths only, and strongly reflect other wavelengths.

Question 11

🟢 How are each colour of visible light fit into the light spectrum

Answer 11

Each colour in the visible light spectrum has its own narrow band of wavelength and frequency. Each side of each band merges into the adjacent bands.

Question 12

❌ How do Colour Filters work

Answer 12

Colour filters work by absorbing certain wavelengths and transmitting other wavelengths. For example, if white light is directed at a red filter, the filter transmits only red light because it absorbs all the parts of the white light spectrum except for red.

Red, green, and blue are called the primary colours of light because they can be mixed to produce any other colour of light.

Question 13

❌ Why does some light look red to us, while some looks blue or green or pink, etc?

Answer 13

Light is a type of electromagnetic wave.

The only difference between the different colours of light are the wavelength/frequency of the electromagnetic wave.

Question 14

❌ How are Rainbows formed?

Answer 14

White light is made of all the colours of the spectrum

We can split white light into the colour spectrum using a prism

Question 15

❌ How does a Prism split white light

Answer 15

The different wavelengths (i.e colours) of light travel at different speeds through the glass of the prism, so they get refracted by different amounts, which allows the prism to split white light into the different colours of the spectrum. When light is split up and separated colours it is said to have been dispersed.

Question 16

❌ How will a red filter transmit visible light

Answer 16

Colour filters work by absorbing certain wavelengths and transmitting other wavelengths.

A red filter will transmit red light amd absorb all other wavelengths of light.

Question 17

❌ How will a green filter transmit visible light

and

How will a green filter transmit red light

Answer 17

A green filter will transmit green light and absorb all other wavelengths of light.

A green filter will absorb the red light and no other light will be tramitted at all.

Question 18

❌ Definition of Transparent objects

Answer 18

Transmits all light through the object. e.g. a clear glass window.

Question 19

❌ Definition of Translucent objects

Answer 19

Light is transmitted, but is scattered or refracted as it does so, due to lots of internal boundaries in the object. e.g. a frosted window.

Question 20

❌ Definition of Opaque objects

Answer 20

Light does not pass through. Instead the object either absorbs all light that reaches it, or reflects / scatters it at the surface.

Question 21

❌ Why is a glass window transparent

Answer 21

All wavelengths of light is absorbed, and all light is transmitted

Question 22

❌ Why is frosted glass translucent

Answer 22

All wavelengths of light are absorbed, and only some wavelengths of light is transmitted and some is reflected.

Question 23

❌ Why is a wooden table opaque

Answer 23

All wavelengths of light are absorbed, and all wavelengths of light are reflected

Question 24

❌ What are the rules for coloured light going through filters

Answer 24

• If the object is the same colour as the filter you would see that colour.
• If the object is white you would see the colour of the filter
• If the object is a different colour to the filter it would appear to be black because no light is reflected

Question 25

❌ A blue object viewed through a red filter will look __________________. This is because the blue object can only _________________ blue light, and the red filter only ______________ red light.

Answer 25

A blue object viewed through a red filter will look black. This is because the blue object can only reflect blue light, and the red filter only transmits red light.

Question 26

❌ The lights at the party turn red. Explain how the colour of the T-shirt changes.

Answer 26

The T-shirt will change from blue to appearing black, because the T-shirt absorbs red light/does not reflect red light

Question 27

❌ What do lenses do (how do lenses work)

Answer 27

A lens works by changing the direction of light passing through it. The curved shape of a lens surface refracts the rays so they meet ata point. Each section of the lens refracts light as it goes in, and again as it comes out. The overall effect is to make the light rays converge.

Question 28

❌ What do Convex Lenses and Concave Lenses do

Answer 28

Different lens shapes can be tested using this arrangement.

• A convex lens (or converging lens) makes parallel rays converge to a focus. The point where parallel rays are focused to is the principal focus of the lens. A converging lens is used as a magnifying glass and in a camera to form a clear image of a distant object
• A concave lens (or diverging lens) makes parallel rays diverge (spread out). The point where the rays appear to come from is the principal focus of the lens. A diverging lens is used to correct short sight.
• Whether the lens is concave or convex, the distance from the centre of the lens to the principal focus is called the focal length of the lens. In ray diagrams, the principal focus is usually shown on each side of the lens.

Question 29

❌ How do find a real and virtual image on a Convex Lens

Answer 29

1. With the object at different distances beyond the principal focus of the lens, adjust the position of the screen until you can see a clear image of the object on it. This is a real image because it can be formed in a screen.
2. With the object nearer to the lens than the principal focus, a magnified image is formed. The image is a virtual image because it is formed where the rays appear to come from. In this situation, the lens acts as a magnifying glass.

Question 30

❌ Where is the image formed when the distance of an object is near or far for a Convex Lens

Answer 30

• When the object is a long distance away, the image is formed at the principal focus of the lens. This is because the rays from any point on the object are almost parallel to each other when they reach the lens
• If the object is moved nearer to the lens towards its principal focus, the screen must be moved further from the lens for you to see a clear image. The nearer the object is to the lens, the larger the image.

Question 31

❌ Magnification Equation, how can you tell where the image is bigger than the object, and what unit does it have

Answer 31

The magnification produced by the lens = Image Height/Object Height

If the image is bigger than the object, the magnification is greater than 1. If the image is smaller than the object, the magnification is less than 1.

Magnification is a ratio, so it does not have a unit. Image height and object height should both be measured in the same unit - either millimeters or centimeters.

Question 32

❌ What do the position and nature if the image formed by a lens depend on

Answer 32

The poition and nature of the image formed by a lens depends on:

• The focal length, f, on the lens
• The distance from the object to the lens

If you know the focal length and the object’s distance, you can find the position and nature of the image by drawing a ray diagram.

Question 33

❌ How is a real image formed by a convex lens

Answer 33

To form a real image using a convex lens, the object must be beyond the principal focus, F, of the lens. The image is formed on the other side of the lens to the object.

Question 34

❌ How do you locate the image and determine if’s nature

+

How does light act along the different ray paths to form a real image by a convex lens

Answer 34

To locate the image and determine its nature, three main rays are used as construction lines from a single point of the object.

• The principal axis of the lens is the straight line that passes along the normal at the centre of each lens surface. The lens is drawn as a straight line with outward arrows to show that it is a convex lens.
• The image is real, inveryed, and smaller than the object

Light acts along the different ray paths:

• Ray 1 is refracted through F, the principal focus of the lens, beacuse it is parallel to the principle axis of the lens before it passes through the lens
• Ray 2 passes through the centre of the lens (its pole) without a change in direction - this is because the lens surfaces at the principal axis are parallel to each other
• Ray 3 passes through F, the principal focus of the lens, before it reaches the lens, so it is refracted by the lens parallel to the principal axis.

Question 35

❌ How is a real image produced in a camera (for both near and distant objects, also, what lens is used)

Answer 35

In a camera, a convex lens is used to produce a real image of an object on a film (or on any array of pixels for a digital camera). The position of the lens is adjusted to focus the image on the film.

• For a distant object, the distance from the lens to the film must ge equal to the focal length of the lens
• The nearer an object is to the lens, the bigger the distance is from the lens to the film.

Question 36

❌ How is a virtual image formed by a convex lens

Answer 36

The object must be between the lens and its principal focus. The image on the same side of the lens as the object.

The image is virtual, upright, and larger than the object.

The image can be seen only by looking at it through the lens. This is how a magnifying glass works.

Question 37

❌ For is a virtual image formed by a concave lens

Answer 37

The image formed by a concave lens in always virtual, upright, and smaller than the object. A concave lens is shown as a line with an inwards arrow.

Question 38

❌ Light waves have a wavelength of 5x10^–7 m and a frequency of 600x10^12 Hz. What is the speed of light?

Answer 38

Magnification = Image Height / Object Height

10 / 5 = 2

The magnification is 2

Question 39

❌ Light waves have a wavelength of 5x10^–7 m and a frequency of 600x10^12 Hz. What is the speed of light?

Answer 39

Wave Speed = Frequency x Wavelength

5x10^-7 x 600x10^12 = 3 x 10^8 m/s

Question 40

❌ The speed of light calculated by a student was 3x10^5 km/s. The wavelength was 1x10^–3 m. Calculate the frequency of light.

Answer 40

Frequency = Wave Speed / Wavelength

(1km = 1000m
1km = 1x10^3m)

3x10^8 / 1x10^-3 = 3x10^11 Hz

Question 41

❌ Light waves with a frequency of 7.26x10^14 Hz lie in the ultra-violet region of the visible spectrum. What is the wavelength of this frequency of light?

(Assume Wave Speed = 3x10^8 m/s)

Answer 41

Wavelength = Wave Speed/Frequency

3x10^8 / 7.26x10^14 = 4.13x10^-7 Hz

Question 42

🟢 What is Refraction?

Answer 42

Refraction refers to the changing direction of a wave (light) as it travels between mediums of different densities.

Question 43

🟢 How does the angle of incidence compare to the angle of refraction when a wave passes into a denser medium?

Answer 43

The angle of refraction will be smaller than the angle of incidence, due to the fact that light is refracted towards the normal when it passes from a less dense medium to a more dense medium.

Question 44

🟢 How does the angle of incidence compare to the angle of refraction when a wave passes in a less dense medium?

Answer 44

The angle of refraction will be larger than the angle of incidence, due to the fact that light is refracted away from the normal when it passes from a more dense medium to a less dense medium.

Question 45

❌ How can Convex Lens be represented on Ray Diagrams

Answer 45

In ray diagrams, a convex lens can be represented by:

   /|\
    |
    |
    |
    |
    |
   \|/

Question 46

❌ How can Concave Lens be represented on Ray Diagrams

Answer 46

In ray diagrams, a concave lens can be represented by:

    \/
    |
    |
    |
    |
    |
    |
    |
    /\

Question 47

❌ What is the Principle Focus (focal point) of a lens

Answer 47

The principal focus, F (focal point), of the lens is the point where rays appear to focus.

Question 48

❌ What is the Focal Length (focal point) of a lens

Answer 48

Focal length refers to the distance between the centre of the lens and principal focus.

Question 49

❌ Convex Lens Definition

Answer 49

A convex lens makes parallel rays converge (focus) towards the centre of the lens.

Question 50

❌ Concave Lens Definition

Answer 50

A concave lens makes parallel rays diverge away (spread out) from the centre of the lens.

Question 51

❌ Example Question:

A cheek cell is 60 μm in diameter, what magnification is needed to provide an image with a cell diameter seen as 12mm?

Answer 51

1200μm / 60μm = 200

200 magnification

Question 52

❌ What happens in Myopia (Short-Sightedness)

Answer 52

Ray focuses in front of the retina. This shows short sighted vision (near sighted)

Question 53

❌ What happens in Hyperopia (Long-Sightedness)

Answer 53

Ray focuses in behind of the retina. This shows long sighted vision (long sighted)

Question 54

❌ Find the Type of Image and Size of Image (Bigger/Smaller than object) when:

Distance from lens to object = Greater than 2F
Distance from lens to image = Between 2F and F

Answer 54

Type of Image:
Real, Inverted

Size of Image:
Smaller than object

Question 55

❌ Find the Type of Image and Size of Image (Bigger/Smaller than object) when:

Distance from lens to object = Equal to 2F
Distance from lens to image = Equal to 2F

Answer 55

Type of Image:
Real, Inverted

Size of Image:
Equal to object

Question 56

❌ Find the Type of Image and Size of Image (Bigger/Smaller than object) when:

Distance from lens to object = Between 2F and F
Distance from lens to image = Greater than 2F

Answer 56

Type of Image:
Real, Inverted

Size of Image:
Larger than object

Question 57

❌ Find the Type of Image and Size of Image (Bigger/Smaller than object) when:

Distance from lens to object = Less than F
Distance from lens to image = Greater than 2F

Answer 57

Type of Image:
Virtual, Upright

Size of Image:
Larger than object

Question 58

🟢 Describe what may happen when waves cross the boundary from air to water.

Answer 58

When waves cross the boundary from air to water, the waves travel from a rarer medium to a more dense medium. When a waves travels into a more dense medium, the waves are refracted towards the normal, and the wavelength and wave speed both decrease (and the waves lose energy from the boundary). Also, some waves may also be reflected as well as refracted.

Question 59

❌ An image with a height of 7.5 cm is produced by an object with a height of 3 cm.

Calculate the magnification of the object. Use the equation:

Magnification = Image Height Object / Height

Answer 59

7.5 / 3 = 2.5

Magnification = 2.5

Question 60

❌ Explain why a red coat with a green collar looks its true colours when seen in white light.

Answer 60

White light is composed of all colours in the spectrum, and the red and green parts of light will be reflected from the coat, and the other colours are absorbed and not reflected by these parts of the coat.