Topic 5 - Light and the electromagnetic spectrum Flashcards
Explain reflection
The angle of incidence = the angle of reflection
Angles are always measured from the normal.
Waves reflect at the boundary between 2 different materials.
Explain refraction
Refraction is the change in direction of a wave at a boundary between 2 transparent materials.
If entering a denser material, it bends towards the normal. If entering a less dense material, it bends away from the normal.
Explain total internal reflection
When a light ray is passing from a denser medium to a less dense medium and the ray is brought out at more than 90° from the normal the refraction is not possible. Therefore, the ray is reflected inside the more dense medium following the law of reflection. The angle of incidence when the ray changes from just refraction to total internal reflection is called the critical angle.
If the angle of incidence is equal to the critical angle, the reflected ray will pass along the boundary and not exit the medium
For glass to air how does light react?
-If the angle is less than the critical angle, light refracts away from the normal
-If the angle is equal to the critical angle, light passes along the boundary
-If the angle is more than the critical angle, light reflects
Describe specular reflection
In specular reflection, waves are reflected from a smooth, flat surface and all exit at the same angle. The image is upright and virtual.
Describe diffuse reflection
When light hits a rough surface, diffuse reflection occurs. Instead of forming an image the reflected light is scattered in all directions. This may cause a scattered or no image.
Describe colours in terms of light
Each colour within the visible light spectrum (ROYGBIV) has its own narrow band of wavelength and frequency. ROYGBIV goes in order of increasing frequency and decreasing wavelength.
An object appears to be black if it absorbs all the wavelengths of visible light (e.g. a blue object in red light).
White light reflects all colours
Explain how colour of light is related to differential absorption at surfaces
Waves can be absorbed at the boundary between 2 different materials. When waves are absorbed by a surface, the energy of the wave is transferred to the particles in the surface. This 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.
Explain transmission of light waves
Waves can be transmitted at the boundary between 2 different materials. When waves are transmitted the wave continues through the material. Transparent materials are very good at transmitting light with very little absorption. Translucent materials transmit some light
Explain how colour of light is related to the transmission of light through filters
When white light passes through a coloured filter, all colours are absorbed except for the colour of the filter.
What is focal length?
The distance between the lens and the focal point
What is the focal point?
The point where all horizontal rays meet after passing through the lens
How do you work out the power of a lens?
Power of the lens is the inverse of the focal length.
e.g. Thicker lens means shorter focal length, so greater power
Describe concave lenses
A concave lens is thinner in the middle than it is at the edges. This causes parallel rays to diverge and spread light outwards. Light appears to have come from the focal point.
In a ray diagram, a concave lens is drawn as a vertical line with inward pointing arrows either end to indicate the shape of the lens.
Describe how concave lenses can correct eyesight
It is used to spread light out further. Concave lenses correct short-sightedness (myopia) as light is focused in front of the retina, so needs to be spread out slightly to be able to be focused onto the retina.
How do you draw the image of an object through a concave lens?
-Draw a horizontal ray from the top of the object to the lens. This will be refracted:
* Draw a line from F (on the side of the object) through the top of the lens
-Draw a line from the top of the object through the centre of the lens
-The image is where the lines intersect
Describe convex lenses
They are fatter at the centre and focus light inwards. Horizontal rays focus onto the focal point.
In a ray diagram, a convex lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.
Describe how convex lenses can be used
They are used for magnifying glasses, binoculars and to correct long-sightedness as it focuses the rays closer.
How do you draw the image of an object through a convex lens?
-Draw a horizontal ray from the top of the object to the lens. This will be refracted:
* Draw a line from the top of the lens through F (on the opposite side of the lens to the object)
-Draw a line from the top of the object through the middle of the lens.
-The image is where the lines intersect
-The ray exits the lens along a line from the top of the image to the top of the lens
Define each descriptor for an image
-An image may be upright or inverted (upside down compared to the object)
-An image may be magnified or diminished (smaller than the object)
-An image may be real (can be projected onto a screen) or virtual (appears to come from behind the lens)
What images do diverging (concave) lenses produce?
Virtual, upright and diminished images
What images do converging (convex) lenses produce when the object is further from the lens than 2 focal lengths?
An inverted, diminished, real image.
This is used for a camera or human eye.
What images do converging (convex) lenses produce when the object is between 1 and 2 focal lengths from the lens?
AN inverted, magnified, real image.
This is used for projectors.
What images do converging (convex) lenses produce when the object is placed less than 1 focal lengths from the lens?
An upright, magnified, virtual image.
This is used for magnifying glasses.
Describe electromagnetic waves
All electromagnetic waves are transverse and travel at the same speed in a vacuum
Explain how all electromagnetic waves transfer energy
All electromagnetic waves transfer energy from source to observer. The waves contain energy e.g. microwaves transfer energy from the source to the food
What is the issue with how we perceive electromagnetic radiation?
Our eyes can only detect a limited range of frequencies of electromagnetic radiation, such as visible light
Recall the main groups of electromagnetic waves in order and describe their order
Continuously from radio waves to gamma rays, in order of increasing frequency and decreasing wavelength:
-Radio waves
-Microwaves
-Infrared
-Visible
-Ultraviolet
-X-rays
-Gamma rays
Describe the core practical to investigate refraction in rectangular glass blocks in terms of the interaction of electromagnetic waves with matter
-Set up a ray box, slit and lens so that a narrow ray of light is produced.
-Place a ruler near the middle of a piece of plain paper. Draw a straight line parallel to its longer sides. Use a protractor to draw a second line at right angles to this line. Label this line with an ‘N’ for normal.
-Place the longest side of a rectangular acrylic polymer block against the first line. With the normal near the middle of the block, carefully draw around the block without moving it.
-Use the ray box to shine a ray of light at the point where the normal meets the block. This is the incident ray.
-The angle between the normal and the incident ray is called the angle of incidence. Move the ray box or paper to change the angle of incidence. The aim is to see a clear ray leaving the opposite face of the block.
-Using a pencil on the paper, mark the path of the incident ray with a cross and the ray that leaves the block with a cross at the point the ray leaves the block.
-Remove the block. Draw a line joining each cross to the point where the normal meets the block to show the paths of the light rays.
Repeat steps 2 to 7 for different angles of incidence.
-Measure the angle of incidence and angle of refraction for each block.
How do substances react to electromagnetic waves?
Different substances may absorb, transmit, refract or reflect electromagnetic waves in ways that vary with wavelength
Explain the effects of differences in the velocities of electromagnetic waves in different substances
When a wave travels across a boundary, it will change velocity (as the densities of the 2 substances are different). As the waves travel at different velocities in different substances, this can lead to the wave changing direction – which is refraction.
Describe the relationship between temperature and radiation
All bodies emit radiation. As temperature increases, the wavelength decreases and the intensity increases.
How can a body retain a constant temperature?
For a body to be at a constant temperature it needs to radiate the same average power that it absorbs
What happens to a body if the average power it radiates is less than the average power that it absorbs?
If it emits less power than it absorbs, the temperature will increase
What happens to a body if the average power it radiates is more than the average power that it absorbs?
If it emits more power than it absorbs, the temperature will decrease
Describe the relationship that explains how radiation affects the temperature of the Earth
The temperature of the earth is maintained by the amount of energy received and emitted from the sun.
Explain how the temperature of the Earth is affected by factors controlling the balance between incoming radiation and radiation emitted
-Visible light and short wavelength, high frequency infra-red radiation from the sun is absorbed by the surface of the Earth (some is reflected by the atmosphere) and the planet’s internal energy increases and the surface gets hotter. Some of this energy is transferred to the atmosphere by conduction and convection.
The earth radiates lower frequency infra-red radiation, some of which is absorbed by greenhouse gases in the atmosphere which keeps the earth warm. The greenhouse gases emit infrared radiation, some towards Earth, which is then reabsorbed.
Describe the core practical to investigate how the nature of a surface affects the amount of thermal energy radiated or absorbed
-Place a Leslie cube on a heat-resistant mat. Almost fill it with boiling water and replace the lid.
-Leave for one minute. This is to enable the surfaces to heat up to the temperature of the water.
-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.
(One surface is matt black, another matt white, another shiny black, another shiny silver)
-Plot a bar chart of Infrared radiation (W/m²) on the y-axis to Type of surface on the x-axis
What is the relationship associated with the potential danger of an electromagnetic wave?
The potential danger associated with an electromagnetic wave increases with increasing frequency
What is the harmful effect on people of excessive exposure to microwaves?
The internal heating of body cells
What is the harmful effect on people of excessive exposure to infrared radiation?
Skin burns
What is the harmful effect on people of excessive exposure to ultraviolet radiation?
Damage to surface cells and eyes, leading to skin cancer and eye conditions
What is the harmful effect on people of excessive exposure to x-rays and gamma rays?
Mutation or damage to cells in the body
Describe some uses of radio waves
Broadcasting, communications and satellite transmissions as they are transmitted easily through air, do not cause damage if absorbed by the human body and can be reflected to change their direction.
Describe some uses of microwaves
Cooking, communications and satellite transmissions.
Cooking - They have frequencies which are easily absorbed by molecules in food and the internal energy of the molecules increase when they absorb microwaves.
Communications, satellite transmissions - Microwaves also pass easily through the atmosphere, so they can pass between stations on Earth and satellites in orbit.
Describe some uses of infrared radiation
Cooking, thermal imaging, short range communications, optical fibres, television remote controls and security systems.
Cooking - It has frequencies which are absorbed by some chemical bonds and the internal energy of the bonds increases when they absorb infrared light, which causes heating.
Thermal imaging - The human eye cannot see this light. All objects emit infrared light. Infrared cameras can detect it.
Describe some uses of visible light
Vision, photography and illumination. It is the light that we can see.
Describe some uses of ultraviolet radiation
Security marking, fluorescent lamps, detecting forged bank notes and disinfecting water.
Fluorescent lamps, detecting forged bank notes - Fluorescent substances absorb ultraviolet light produced and re-emit the energy as visible light.
Disinfecting water - The hazardous properties of UV mean it will kill bacteria.
Describe some uses of x-rays
Observing the internal structure of objects, airport security scanners and medical x-rays.
Medical x-rays - They are transmitted through body tissues with very little absorption but are absorbed by dense structures like bones allowing them to identify broken bones.
Describe some uses of gamma rays
Sterilising food and medical equipment, and the detection of cancer and its treatment.
Gamma rays are produced by changes in the nucleus of an atom and form in nuclear radiation. They are transmitted through body tissues with very little absorption.
Describe radio waves connection to electrical circuits
Radio waves can be produced by, or can themselves induce, oscillations in electrical circuits
What can changes in atoms and nuclei do?
Changes in atoms and nuclei can generate radiations over a wide frequency range and be caused by the absorption of a range of radiations
