Topic 5 - Light and the electromagnetic spectrum

Question 1

Explain reflection with the law of reflection

Answer 1

normal line drawn at 90 degrees to mirror surface at point of arriving or incident ray meets the mirror. The angles I and R (incident, reflected ray) measured from this line

The law of reflection is angle i = angle r

Question 2

Describe the process of refraction

Answer 2

Refraction- change in direction light ray. happens when travels from one material into another.

ray of light bends towards normal as enters the glass and away when leaves

Rays of light that meet a surface at 90 degrees dont bend but continue into material without changing direction

Question 3

Explain why refraction occurs

Answer 3

light waves travel at different speeds in different materials.

eg / light waves travel slower in glass than air

Question 4

Explain the role of refraction in total internal reflection

Answer 4

Refraction is when light slows down and changes direction when it travels from less to more dense medium.

Light slows down and bends towards normal when moving from air into glass or water.

Question 5

Explain the role of reflection in total internal reflection

Answer 5

As angle of incidence increases, angle of refraction increase until reaching 90 degrees.

Here, ray of light travelling along outer surface of glass.

The angle of incidence = CRITICAL ANGLE.

Above critical angle, light is totally internally reflected

Question 6

Explain total internal reflection and critical angles

Answer 6

Total internal reflection - light travelling dense material meets a boundary less dense material

Light speeds up and changes direction away from the normal when it travels from glass into air.

Total internal reflection can take place with sound as well as light.

In total internal reflection angle i > critical angle c

Question 7

Explain the difference between specular and diffuse reflection

Answer 7

SPECULAR REFLECTION - occurs when waves are reflected from a smooth surface. When parallel rays of light are incident on a smooth, plane surface eg a mirror, the reflected rays will also be parallel

DIFFUSE REFLECTION - occurs when the surface isnt smooth and has rough irregularities. The size of the irregularities is comparable with the wavelength of the wave. The incident wave is then reflected at many different angles and the reflected rays will not be parallel, such as when light i reflected off a painted wall

Question 8

Relate the power of a lens to its focal length and shape

Answer 8

The greater the power of a lens, the more it bends light. This is related to the shape of the lens

Question 9

Explain the effects of different types of lens in producing real and virtual images

Answer 9

A REAL IMAGE is an image that can be produced on a screen. The lens focuses light rays at the screen. It is formed where light rays converge and are actually focused on a screen.

A VIRTUAL IMAGE is formed by a converging lens when the objet is between the focal point and the lens. A diverging lens provides a virtual image. This is formed by light rays which appear to diverge from that point, but don’t actually do so.

Question 10

Explain the focal point in relation to a converging lens

Answer 10

parallel light rays from object pass through converging lens, brought to focus at point - the focal point or principal focus.

object ‘at infinity‘ if its distance from lens is greater than focal length of the lens

Question 11

Explain the nature of a real image

Answer 11

real image is upside down and smaller than object.

Moving object closer to lens causes image to grow. position of image will move away but will remain real and upside down.

At a distance twice focal length (2F) object and image are same size. Between F and 2F, image is magnified

Question 12

Explain the properties of electromagnetic waves

Answer 12

• they are all TRANSVERSE waves (the em vibrations are at right angles to the direction the wave is travelling)
• travel at the same speed (3 x 10^8 m/s) in a vacuum
• transfer energy to the observer

Question 13

Explain the properties of electromagnetic waves

Answer 13

• they are all TRANSVERSE waves (the em vibrations are at right angles to the direction the wave is travelling)
• travel at the same speed (3 x 10^8 m/s) in a vacuum
• transfer energy to the observer
• they are composed of an electric field and a magnetic field at right angles to one another

Question 14

Explain precautions needed to be taken when investigating refraction in rectangular glass blocks in terms of the interaction of electromagnetic waves with matter

Answer 14

• when carrying out the investigation, make sure that you direct a thin beam of light towards the point at which the normal makes contact with the glass block. Clearly mark the angles from 0 - 80 with a sharp pencil and ruler
• remember the angle of incidence is measured with respect to the normal line
• light will slow down more when it travels from air into glass than it will when it travels from air into water. This is because the ‘optical density’ of glass is greater than that of air

Question 15

List the electromagnetic waves in order of longest wavelength and lowest frequency first

Answer 15

Radio
Microwave 
Infrared
Visible
Ultraviolet
X rays
Gamma 

(Red monkeys in vans use x ray glasses)

Question 16

Describe the electromagnetic spectrum

Answer 16

the electromagnetic spectrum is continuous from radio waves to gamma rays and that the radiations within it can be grouped in order of decreasing wavelength and increasing frequency.

AS THE FREQUENCY OF THE RADIATION INCREASES, THE WAVELENGTH DECREASES

Question 17

Which em frequencies can our eyes pick up on

Answer 17

Our eyes detect the part of the electromagnetic spectrum that is visible light, but there are other wavelengths that behave differently.

Question 18

Explain the behaviour of em waves

Answer 18

Electromagnetic waves may be:

• reflected off a surface
• refracted when the move from one material to another
• transmitted when they pass through a material
• absorbed by different materials. For example, UV is absorbed by the skin but not by earths atmosphere

The extent to which these four things happen depends on the material and the wavelength of the EM waves

Question 19

Explain the effects of differences in the velocities of electromagnetic waves in different substances

Answer 19

Electromagnetic waves have different velocities in different materials. This is linked to an extent to the density of the material.

The speed of light in a diamond is around 40% of the speed of light in a vacuum or air, due to the high optical density of diamond

Question 20

Explain how all bodies emit radiation

Answer 20

All objects above absolute zero will emit electromagnetic radiation. Their temperature will depend on how much of it they absorb and how of it they radiate .

Question 21

Explain how all bodies emit radiation

Answer 21

All objects above absolute zero will emit electromagnetic radiation. Their temperature will depend on how much of it they absorb and how of it they radiate .

Bodies that are hotter than the sun will emit x rays

Question 22

Explain the factors affecting temperature of the body when emitting radiation

Answer 22

1. A body at a constant temperature absorbs the same amount of radiation as it emits
2. An object will increase its temperature if it absorbs more radiation than it emits
3. An object will decrease its temperature if it emits more radiation than it absorbs

As the temperature of the bod increases, more energetic radiation is emitted including infrared, visible and UV

Question 23

Explain the factors affecting temperature of the earth

Answer 23

The temperature of the earth depends on how much radiation is absorbed and how much is emitted

Question 24

Explain how to investigate the nature of a surface affects the amount of thermal energy radiated or absorbed

Answer 24

METHOD 1:

1. Fill leslies cube (shows how the nature of a surface affects the rate at which thermal energy is absorbed or emitted) with hot water a known temperature. Place a thermometer a small distance away from each of the sides. Plug the top of the cube with a bung
2. Measure the temperature at a distance of 10cm from each of the four sides of leslies cube for a period of 5 minutes, taking a reading every 30 seconds.

METHOD 2:

1. Fill leslies cube with the same volume of cold water at a known temperature. Plug the top of the cube with a thermometer and bung
2. Heat each side, one at a time, with a radiant heater from the same distance away (about 10 cm)
3. Record how the temperature of the water changes, every 30 seconds, over an appropriate period of time k

Question 25

Explain precautions that can be taken when investigating the nature of a surface affects the amount of thermal energy radiated or absorbed

Answer 25

• be careful when using hot water as it may scald
• conduct the investigation accurately and fairly by ensuring that the thermometer used Is the same for each surface, the distance from the surface is the same and the starting temperature is the same in each case. Take the same number of readings over the same time period and intervals

Question 26

Explain how the danger of an electromagnetic wave can increase

Answer 26

the potential danger associated with an electromagnetic wave increases with increasing frequency

Question 27

Explain changes in atoms and nuclei

Answer 27

Changes in atoms and nuclei can:

• generate radiations over a wide frequency range
• be caused by absorption of a range of radiations

Question 28

Explain the effect of radio waves on electrical circuits

Answer 28

radio waves can be produced by, or can themselves induce, oscillations in electrical circuits