P14: Light

Question 1

Describe a method for an investigation into the reflection of light and angles of incidence and reflection

Answer 1

1. Place the mirror on your sheet of paper and mark it with a pencil.
2. Shine the ray of light on the mirror and mark the path the light ray takes.
3. Draw the normal where the light ray meets the mirror.
4. Measure the angle of reflection and the angle of reflection.
5. Repeat at least two more times changing the angle of incidence.

Question 2

What is the law of reflection?

Answer 2

The angle of incidence = the angle of reflection

Question 3

What is a real image?

Answer 3

Real image: An image formed by a lens that can be projected on a screen.

Question 4

What is a virtual image?

Answer 4

Virtual image: An image, seen in a lens or a mirror, from which light rays appear to come after being refracted by a lens or reflected by a mirror. It cannot for, an image on a screen

Question 5

How is the image in a mirror virtual?

Answer 5

The image produced in a plane mirror is virtual, upright and laterally inverted (back to front but not upside down).
* The light race do not actually converge behind the mirror so they only appeared to do so.
* Your brain interprets the reflected light race which seems to be diverging from a point behind the mirror as if they were coming from a real object located there.

Question 6

What is the difference between real and virtual images?

Answer 6

Virtual image can’t be projected onto a screen like the movie images you can see at a cinema.
A ‘real image’ can be seen on a screen and is formed by focusing light rays onto a screen

Question 7

What reflection occurs on a smooth surface?

Answer 7

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 8

What reflection occurs on a rough surface?

Answer 8

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

Question 9

Write a method for a practical investigating refraction of light

Answer 9

1. Place the glass or Perspex block in the centre of the piece of paper (landscape view) and draw around its edges.
2. Remove the block and draw a dotted normal line at right angles to the edge of one long side of the block, about half way along.
3. Draw a line to represent the incident ray at 30° to the normal.
   4 Place the block back on the rectangle you have drawn.
4. Switch on the ray box with a narrow slit in front of the bulb and shine the ray along the incident ray you have drawn.
5. Observe the path of the light through the block and out of the other side.

Question 10

Why does light refract the way it does as it moves from water to air?

Answer 10

• As the light wave moves into different densities at an angle, parts of the wave begin to move a different speeds, causing the wave to refract and for its direction to change.
• Moving into a lower density, the side of the ray in which direction it is moving (eg: moving right, right side), will accelerate first, causing the ray to increase the angle of its direction (eg: if originally moving right, the ray will change direction to move right at a greater degree)

Question 11

What is the difference between red light and violet light?

Answer 11

Red light has a greater wavelength and thus a lower frequency than violet light

Question 12

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

Answer 12

When white light is directed towards a certain colour such as red, the colour will only reflect red light as it absorbs all parts of the white light spectrum except for red as it will only reflect the wavelength associated with the colour red

Question 13

How does a prism split white light?

Answer 13

• A prism separates white light into its constituent colors (a spectrum) through a process called dispersion.
• This happens because white light is a mixture of different colors, each with a slightly different wavelength.
• When white light enters a prism, it is refracted (bent) at different angles depending on the wavelength. Shorter wavelengths, like violet and blue, are bent more, further from the normal, than longer wavelengths, like red and orange, which move toward from the normal resulting in the separation of colours.

Question 14

What type of wave is light?

Answer 14

Transverse, electromagnetic wave

Question 15

Which colour has the shortest wavelength?

Answer 15

Violet

Question 16

What colour has the longest wavelength?

Answer 16

Red

Question 17

How does a colour filter work?

Answer 17

Colour filters work by absorbing certain wavelengths and transmitting other wavelengths.
Eg: A red filter will transmit red light and absorb all other wavelengths of light.

Question 18

What would happen if red light reached a green filter

Answer 18

If red light hits a green filter, no light will be transmitted, and the filter will appear black. Green filters absorb red light, and since red light is the only light source, it will be entirely absorbed, resulting in no light being transmitted through the filter.

Question 19

What is a transparent object?

Answer 19

Transmits all light through the object (eg: clear glass window)

Question 20

What is a translucent object?

Answer 20

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 21

What is an opaque object?

Answer 21

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

Question 22

Describe the colours of opaque objects and how they affect the reflection of light

Answer 22

● Chemicals called pigments in the surface of materials determine an item’s colour.
● Colour also depends on the range of the wavelengths in the incident light.
● The pigments absorb certain wavelengths of light and strongly reflect others.
● White surface has no pigments so it reflects any light.

Question 23

What are rules to coloured light and coloured objects?

Answer 23

• 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 24

What is a lens?

Answer 24

A lens forms an image by refracting light

Question 25

What is refraction?

Answer 25

Refers to the change in direction and speed of light waves as it changes medium and density, altering its distance from the normal line

Question 26

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

Answer 26

The angle of refraction will be smaller than the angle of incidence as the wave moves towards the normal line

Question 27

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

Answer 27

The speed will increase and the wave will move away from the normal line. The angle of refraction will be greater than the angle of incidence

Question 28

How is a convex lens represented in ray diagrams?

Answer 28

Two way arrow

Question 29

How is a concave lens represented in ray diagrams?

Answer 29

Two way arrow but the points are inverted

Question 30

What is a convex lens?

Answer 30

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

Question 31

What is the principle focus point?

Answer 31

The principal focus, F (focal point), of the lens is the point where rays appear to focus- there are always two principal focus points on either side of the lens

Question 32

What is the focal length (convex)?

Answer 32

Focal length refers to the distance between the centre of the lens and principal focus (after the lens)

Question 33

How must a ray diagram be drawn with a convex lens?

Answer 33

Parallel lines towards the lens Once the lines reach the lens, the outer lines are refracted slightly inward and the centre line remains straight After the lens, all rays must be drawn as straight lines, meeting at the principle focus point

Question 34

What is a concave lens?

Answer 34

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

Question 35

What is the focal length (concave)?

Answer 35

Focal length refers to the distance between the centre of the lens and principle focus (before the lens)

Question 36

How must ray diagram be drawn with a concave lens?

Answer 36

- Draw two lines from the first principal focus point to the point where the outer parallel lines meet the lens - Continue this line through the lens until reaching the end - Continue this line out the lens at a slightly smaller angle - Continue the centre parallel line forward to pass the other principal focus point

Question 37

How can magnification be calculated?

Answer 37

Magnification = Image height/ Object height (both must be same units)

Question 38

How to convert 1mm to nanometres

Answer 38

x1000

Question 39

How do the lenses of glasses correct vision?

Answer 39

The reason people are either short or long sighted is because the lens in their eye focuses the light either before or after the back of the retina

Question 40

How does the eye work?

Answer 40

Light reflected from an object and directed to your eyes enters your eye through the pupil. The iris changes the size of the pupil, depending on how bright the light is. The lens focuses the light onto the back of the eye: the retina. The retina is a mass of light-sensitive neurons, called photoreceptors, which change light signals into electrical ones The electrical signals travel via the optic nerve to the brain. Our brains turn the image aroung so we see an upright image.

Question 41

How can a ray diagram be drawn for an object and convex lens to produce an image? (both convex and concave)

Answer 41

Ray 1 – A ray passing through the optical centre, o, of the lens from the object Ray 2 – a ray parallel to the principal axis, which refracts through the lens, passing through the principal focus (F) Draw: Ray 3 – a ray passing through the principal focus (on the same side as the object) and being refracted through the lens, emerging parallel to the principal axis

Question 42

What is the nature of the image formed?

Answer 42

Real or virtual Flipped or upright Is the image smaller or larger than the object- diminished or magnified

Question 43

Hpw to tell of the image is real or virtual

Answer 43

Real: actual ray lines Virtual: Extended dotted ray lines

Question 44

How can short-sightedness be corrected?

Answer 44

Short-sightedness can be corrected by a concave lens, this causes the rays to diverge before passing through the lens. This assists in bringing the rays to converge on the retina rather than in front of the retina.

Question 45

How can long-sightedness be corrected?

Answer 45

Long-sightedness can be corrected by a convex lens, this assists in bring the rays to converge on the retina rather than behind the retina.

Question 46

What type of image does a concave lens form?

Answer 46

A concave lens forms a virtual and upright image, smaller than the object

Question 47

What happens if the lines of a ray diagram are parallel

Answer 47

No image is formed

Question 48

What happens if one of the line passes the lens but not past the line?

Answer 48

Extend the line of the lens

Question 49

What is lens power and its formula?

Answer 49

The more powerful the lens the more strongly it converges/diverges rays of light so the shorter the focal length. P = Power in dioptres (D) f = focal length in meters (m) P= 1/f Converging lens have a positive power, diverging lenses have a negative one.

Question 50

What is total internal reflection?

Answer 50

When light passes through a boundary between two media some is reflected and some refracted. TIR is when all of it is reflected. It can only occur when light travels through a dense material like glass, water or plastic towards a less dense substance like air. Whether TIR occurs depends on the critical angle of the boundary and the angle of the incidence of the light. The critical angle is the angle of incidence above which total TIR occurs. So TIR can only occur when the angle of incidence is bigger than the critical angle.

Question 51

What is the critical angle + link to TIR?

Answer 51

The angle of incidence when the angle of refraction is 90°, is called the critical angle. Total internal reflection only occurs when: light travels from a less dense medium to a more dense medium e.g. from air to glass. the angle of incidence in the more dense medium is greater than the critical angle, when the angle of refraction is above 90° from the normal.

Question 52

What is the relation between the angle of incidence, critical angle and reflection?

Answer 52

- if the angle of incidence is less than the critical angle, the light refracts away from the normal - if the angle of incidence is equal to the critical angle, the light refracts at 90° to the normal - if the angle of incidence is greater than the critical angle, total internal reflection occurs

Question 53

What does the critical angle depend on?

Answer 53

The value of the critical angle of an material-air boundary depends on the refractive index of the material. A dense material with a high refractive index has a low critical angle at is boundary with air. IF a material has a high refractive index, it will totally internally reflect more light at its boundary with air - more light will be incident at an angle bigger than the critical angle.

Question 54

What is the formula for the critical angle?

Answer 54

Refractive angle = 1/ Sin C Where c = critical angle

Question 55

How do optical fibres work?

Answer 55

- Optical fibres carry visible light long distances due to total internal reflection. - They work by bouncing light of the sides of a thin inner core of glass or plastic. - The wave enters one end of the fibre and undergoes TIR repeatedly, as the angle of incidence is repeatedly larger than the critical angle, at the boundary of the inner core until it emerges at the other end. - The inner core needs to be denser than the outer layer so that TIR can happen.

Question 56

What is a laser?

Answer 56

A narrow, intense beam of light. The light waves that come from a laser all have the same wavelength. They can be dangerous at they're powerful but can be used in medicine. For example they can be used to cut through body tissue instead of using a scalpel since they cauterise (burn and seal shut) small blood vessels as they cut through tissue. This reduces the amount of blood the patient loses and helps protect against infections.

Question 57

How else can lasers be medically used?

Answer 57

Lasers can be used to treat skin conditions like acne scars - they can burn of the top layers of scarred skin revealing less-scarred skin. They can also be used in laser eye surgery. The laser is used to vapourise the cornea to shape its shape to change its focusing ability. This can increase or decrease the power of the corean so that the eye can focus images properly on the retina

Question 58

What is the difference between ray diagrams of concave and convex lenses?

Answer 58

For ray diagrams of concave lenses, the focal point is before the lens. The lines drawn diverge away from each other past the concave lens A dashed lone must be drawn backward from these lines at the same angle until they meet to form an image.