Topic 5 - Light and the Electromagnetic Spectrum

Question 1

What is the law of reflection?

Answer 1

Angle of incidence = Angle of reflection

Question 2

How does a wave experience total internal reflection?

Answer 2

This happens when a wave hits a surface so when it travels through a more dense material like glass or water, towards a less dense substance like air.

The angle of incidence must be larger than the critical angle.

+If i is equal to the critical angle then the ray would go along the surface [with quite a bit of internal reflection as well]

+If i is less than the critical angle, most of the light is refracted into the outer layer, but some is internally reflected.

Question 3

What is specular reflection?

Answer 3

This is when the waves are reflected in a single direction by a smooth surface.

Question 4

What is diffuse reflection?

Answer 4

When waves are reflected by a rough surface (such as paper) and the waves are reflected in all directions.

When light is reflected by something rough, the surface looks matt and you don’t get a clear reflection.

This is because the normal is different for each incident ray, so each has a different angle of incidence [the rule of angle of incidence = angle of reflection still applies]

Question 5

Which objects don’t transmit light?

Answer 5

Opaque objects. When visible light hits them, they absorb some wavelengths of light and reflect others.

Question 6

Describe the absorption of light

Answer 6

Waves can be absorbed at the boundary between two different materials.

When waves are absorbed by a surface, the energy of the wave is transferred to the particles in the surface. Will usually increase the internal energy of the particles.

When white light shines on an opaque object, some wavelengths or colours of light are absorbed.

These wavelengths are not detected by our eyes.

The other wavelengths are reflected, and these are detected by our eyes.

Question 7

Describe the transmission of light

Answer 7

Waves can also be transmitted at the boundary between two different materials.

When waves are transmitted, the wave continues through the material.

Air, glass and water are common materials that are very good at transmitting light.

They are transparent because light is transmitted with very little absorption.

Translucent materials transmit some light but are not completely clear.

Lamp shades, shower curtains and window blinds are often translucent objects.

Question 8

What happens when white light passes through coloured filters?

Answer 8

When white light passes through a coloured filter, all colours are absorbed except for the colour of the filter.

For example, an orange filter transmits orange light but absorbs all the other colours.

If white light is shone on an orange filter, only the orange wavelengths will be observed by the human eye.

Question 9

What is a convex lens?

Answer 9

A convex lens is thicker in the middle than it is at the edges.

Parallel light rays that enter the lens converge.

Question 10

What is a concave lens?

Answer 10

A concave lens is thinner in the middle than it is at the edges.

This causes parallel rays to diverge.

They separate, but appear to come from a principle focus on the other side of the lens.

Question 11

What is the axis of the lens?

Answer 11

A line passing through the middle of the lens

Question 12

What is the principal focus of a convex lens?

Answer 12

The point where rays hitting the lens parallel to the axis all meet.

Question 13

What is the principal focus of a concave lens?

Answer 13

The point where rays hitting the lens parallel to the axis appear to come from - you can trace them back until they all appear to meet up at a point behind the lens.

Question 14

What is the focal length?

Answer 14

The distance from the centre of the lens to the principal focus, F

Question 15

What is a REAL image?

Answer 15

When the light rays actually come together to form the image.

The image can be captured on a screen, because the light rays actually meet at the place where the image seems to be; the image formed on the retina.

Question 16

What is a VIRTUAL image?

Answer 16

When the light rays from the object appear to be coming from a completely different place to where they’re actually coming from.

The light rays don’t actually come together at the point where the image seems to be, so it cannot be captured on a screen; magnifying glasses create virtual images.

Question 17

How does the focal length and the type of lens affect the power?

Answer 17

The more powerful the lens, the more strongly it converges a ray of light, so the shorter the focal length.

Convex lens - positive power Concave lens - negative power

Question 18

What types of waves are electromagnetic waves?

Answer 18

Transverse - travel at the same speed through a vacuum.

But they travel at different speeds in different materials. They vary in wavelength 10^-15 to more than 10^4

Question 19

What are electromagnetic waves generated by?

Answer 19

A variety of changes in atoms and their nuclei, giving a large range of frequencies.

Question 20

What is the only part of the EM spectrum we can see?

Answer 20

Visible light - different colours have different wavelengths - from longest to shortest: red, orange, yellow, green, blue, indigo, violet.

Question 21

What do EM waves transfer energy?

Answer 21

All EM waves transfer energy from a source to an absorber.

For example, when you warm yourself by an electric heater, the infrared waves transfer energy from the thermal energy store of the heater (the source) to your thermal energy store (the absorber).

The higher frequency of the EM wave, the more energy it transfers.

Question 22

What is the order of the EM spectrum from long wavelength, low frequency to short wavelength, high frequency?

Answer 22

Radio waves (1 m - 10^4 m);

Micro waves (10^-2 m);

Infrared (10^-5 m);

Visible Light (10^-7 m);

Ultra Violet (10^-8 m);

X-Rays (10^-10 m);

Gamma Rays (10^-15 m)

Question 23

How do radio waves are transmitted, absorbed and reflected have implications on human health?

Answer 23

Radio waves are transmitted through the body without being absorbed therefore it is harmless.

Question 24

How do micro waves that are transmitted, absorbed and reflected have implications on human health?

Answer 24

Some wavelengths of microwaves can be absorbed, causing heating of cells, which may be dangerous.

Question 25

How do Infrared radiation that are transmitted, absorbed and reflected have implications on human health?

Answer 25

Infrared (IR) and visible light are mostly reflected or absorbed by the skin, causing some heating too.

IR can cause burns if the skin gets too hot.

Question 26

How do UV waves that are transmitted, absorbed and reflected have implications on human health?

Answer 26

Ultraviolet (UV) is also absorbed by the skin. But it has a higher frequency, so it is potentially more dangerous.

It’s a type of ionising radiation and when absorbed it can cause damage to cells on the surface of your skin, which could lead to skin cancer.

It can also damage your eues and cause a variety of eye conditions or even blindness

Question 27

How do x-rays that are transmitted, absorbed and reflected have implications on human health?

Answer 27

X-rays and gamma rays are also ionising, so they can cause mutations and damage cells too (which can lead to cancer).

But they have even higher frequencies, so transfer even more energy, causing even more damage.

They can also pass through the skin and be absorbed by deeper tissues.

Question 28

What does the distribution and intensity of EM wave wavelengths depend on?

Answer 28

Temperature. Intensity is the power per unit area (power is energy transferred per second).

As the temperature of an object increases, the intensity of every emitted wavelength increases.

However, the intensity more rapidly for shorter wavelengths than longer wavelengths.

This causes the peak wavelength (the wavelength with the highest intensity) to decrease.

Question 29

What do all bodies emit and absorb?

Answer 29

Infrared radiation. They do this whatever their temperature.

The hotter the body: - the more infrared radiation it gives out in a given time - the greater the proportion of emitted radiation is visible light

Question 30

How is the temperature of your body related to the rate of absorption?

Answer 30

The rate of absorption is greater than the rate of emission Temperature: Increasing

The rate of absorption is equal to the rate of emission Temperature: Constant

The rate of absorption is less than the rate of emission Temperature: Decreasing

Question 31

What affects the Earth’s temperature?

Answer 31

The temperature of the Earth depends on many factors including the concentration of greenhouse gases such as water vapour, methane and carbon dioxide.

The Earth’s temperature also depends on the rates at which light radiation and infrared radiation are:

absorbed by the Earth’s surface and atmosphere

emitted by the Earth’s surface and atmosphere

Question 32

Describe how to investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface. (3)

Answer 32

1) Place a Leslie cube on a heat-resistant mat. Almost fill it with boiling water and replace the lid.
2) Leave for one minute. This is to enable the surfaces to heat up to the temperature of the water.
3) Use the infrared detector to measure the intensity of infrared radiation emitted from each surface, or the temperature of the surface. Make sure that the detector is the same distance from each surface for each reading.

Question 33

What should the results look like for the investigation on how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface.

Answer 33

Surface type

Infrared intensity

matt black 19.5

matt white 5.1

shiny black 14.2

shiny silver 3.8

Question 35

What are radio waves produced by?

Answer 35

Radio waves can be produced by oscillations in electrical circuits.

Question 36

Describe the uses of some EM waves.

Answer 36

Radio waves are used for communication such as broadcasting television and radio, communications and satellite transmissions. Microwaves are used for cooking food, communications and for satellite communications. Infrared (IR) light is used by electrical heaters, cookers for cooking food, short-range communications like remote controls, optical fibres, security systems and thermal imaging cameras which detect people in the dark. Visible light is the light we can see, so is used in photography and illumination. UV - Fluorescent substances are used in energy-efficient lamps - they absorb ultraviolet light produced inside the lamp, and re-emit the energy as visible light. Similar substances are used on bank notes to detect forgeries. The hazardous properties of UV mean it will kill bacteria and can be used for disinfecting water. X-ray imaging is also used for scanning the internal structure of objects and in airport security scanners. Gamma rays are used for sterilising food and medical instruments, and in the treatment and detection of cancer.

Question 37

Desribe the uses of radio waves.

Answer 37

Radio waves are used for communication such as broadcasting television and radio, communications and satellite transmissions.

+It is used for satelites which can pass through the Earth’s watery atmosphere.

+Bluetooth uses short-wave radio waves to send data over distances between devices. [TV and FM radio transmissions have very short wavelengths - to get reception, you must be in direct signal of the transmitter, the signal doesn’t bend or travel far through buildings.

Question 38

How are radio waves made by oscillating charges?

Answer 38

+EM waves are made up of oscillating electric and magnetic fields.

+Alternating currents [a.c] are made up of oscillating charges - as the charges oscillate, they produce electric and magnetic fields, ie. electromagnetic waves.

+The frequency of the waves produced will be equal to the frequency of the alternating current.

+You can produce radio waves using an alternating current in an electrical circuit - the object in which charges [electrons] oscillate to create the radio waves is called a transmitter - when transmitted radio waves reach a receiver, the radio waves are absorbed.

+The energy carried by the waves is transferred to the electrons in the material of the reciever.

+This energy causes the electrons to oscillate and, if the reciever is part of a complete electrical circuit, it generates an alternating current.

+This current has the same frequency as the radio wave that generated it.

Question 39

Describe the uses of microwaves.

Answer 39

Microwaves are used for cooking food, communications and for satellite communications.

+In communications, the microwaves used need to pass through the Earth’s watery atmosphere.

+In microwave ovens, the microwaves need to be absorbed by water molecules in food - so they use a different wavelength to those used in satellite communications.

+The water molecules then transfer this energy to the rest of the molecules in the food by heating - which quickly cooks the food.

Question 40

Describe the uses of infrared.

Answer 40

+Infrared (IR) light is used by electrical heaters, cookers for cooking food, short-range communications like remote controls, optical fibres, security systems and thermal imaging cameras which detect people in the dark.

+The reciever must be in the line of sight of the emitter.

+They use total internal reflection to send lots of data over long distances.

Question 41

Describe the uses of visible light.

Answer 41

Visible light is the light we can see, so is used in photography and illumination.

+Photographic film reacts to light to form an image - this is how traditional cameras create photographs.

+Digital cameras contain image sensors, which detect visible light and generate an electrical signal.

+This signal is then converted into an image that can be stored digitally or printed.

Question 42

Describe the uses of ultaviolet.

Answer 42

UV - Fluorescent substances are used in energy-efficient lamps - they absorb ultraviolet light produced inside the lamp, and re-emit the energy as visible light.

+Similar substances are used on bank notes to detect forgeries. The hazardous properties of UV mean it will kill bacteria and can be used for disinfecting water.

Question 43

Describe the uses of X-rays.

Answer 43

X-ray imaging is also used for scanning the internal structure of objects and in airport security scanners [to detect things that can’t be detected by metal detectors].

+Radiographers in hospitals take X-ray images to help doctors diagnose broken bones - X-rays are transmitted by flesh but are absorbed by denser material like bones or metal.

Question 44

Describe the uses of gamma rays.

Answer 44

Gamma rays are used for sterilising food and medical instruments, and in the treatment and detection of cancer.

+Doctors have to minimise the damage to healthy cells when treating cancer like this.

Question 45

How can you use a glass block to investigate refracton?

Answer 45

+Light is refracted by the boudary between air and glass.

+Place a glass block on a piece of paper and trace around it - use a ray box to shine a ray of light at the middle of one side of the block.

+Trace the incident ray and emergent ray on the other side of the block - remove the block and with a straight line, join up the incident ray and the emergent ray to show the path of the refracted ray through the block.

+Draw the normal at the point where the light ray entered the block - use a portactor to measure the angle between the incident ray and the normal [angle of incidence] and the angle between the refracted ray and the normal [angle of refraction].

+Do the same for the point where the ray emerges from the block.

+Repeat three times, keeping the angle of incidence as the ray enters the enters the block the same. - then calculate an average for each of the angles.