Topic 5 - light and the electromagnetic spectrum Flashcards
Is the angle of incidence equal to the angle of refraction
Yes
What is total internal reflection?
A wave reflecting back into a material
When can total internal reflection happen ?
When a wave travels through a dense material to a less dense material e.g. glass to air
Total internal reflection can only happen when the angle of incidence is ……. than the critical angle
Larger
What happens if (i) is less than the critical angle? (light)
Most of the light is refracted into outer layer but some is internally reflected
What happens if (i) is equal to the critical angle? (light)
The ray would go along the surface (with quite a bit of internal reflection as well)
What happens if (i) is larger than the critical angle? (light)
No light comes out. It’s all internally reflected. (total internal reflection)
What is specular reflection?
When waves are reflected in a single direction by a smooth surface so you get a clear reflection
What is diffuse reflection?
When waves are reflected by a rough surface and the waves are reflected in all directions as the normal is different for each incident ray, so each ray has a different (i)/(r)
Recall practical (refraction)
book
What does colour depend on?
differences in absorption, transmission and reflection of different wavelengths by different materials
How do opaque objects respond to light?
They don’t transmit light. When visible light waves hit them, they absorb some wavelengths and reflect others
Why may an apple appear red?
Because the wavelengths corresponding to the red part of the spectrum are reflected
Why may a banana appear yellow?
Because its reflecting yellow light or its reflecting both green and red light
White objects …..
reflect all wavelengths of light equally
Black objects…
absorb all wavelengths of light. Our eyes see black as the lack of any visible light (colour)
Transparent and translucent objects…
transmit light. These objects appear to be the colour of light that corresponds to the wavelengths most strongly transmitted by the object
What do colour filters only transmit?
Particular wavelengths
If you looked at a blue object through a blue colour filter, what colour would the object look?
blue
If you looked at a red object through a blue filter, what colour would the object look?
black (because all light is absorbed by filter)
What do non-primary filters let through?
The wavelengths of light corresponding to the colour of an object AND the wavelengths of primary colours that can be mixed to make the colour of the object
What is a convex lens?
- A lens that bulges outwards in the middle
2. It causes parallel rays to meet at a principal focus
What is a concave lens?
- A lens that caves inwards
2. It causes parallel rays of light to spread out
What is the axis of a lens?
A line passing through the middle of a lens
What is the principal focus on convex lens?
Where rays hitting the lens parallel to the axis all meet
What is the principal focus on concave lens?
Where rays hitting the lens parallel to the axis appear to have come from - you can trace them back until they all appear to meet up at a point behind the lens
Is there a principal focus on each side of a lens or only on one side?
On each side
What is the focal length?
The distance from the centre of the lens to the principal focus
What are the two types of images that can be formed by lenses?
Real and virtual
What is a real image and give an example?
An image that can be captured on screen because the light rays actually come together to meet at the place where the image seems to be e.g. the image formed on the eyes retina
What is a virtual image and give an example?
An image that cannot be captured on screen because light rays from an object appear to be coming from a completely different place to where they’re actually coming from. The light rays do not come together to meet at the place where the image seems to be. .g magnifying glasses create virtual images
To describe an image, what three things would you need to include in your answer?
- Whether its bigger or smaller than the object.
- If its upright or inverted
- Whether its real or virtual
If an image is inverted, is it a real or virtual image?
A real image
The more powerful a lens, the more strongly it converges rays of light, so the ……… the focal length
(longer, shorter)
shorter
How can you make a lens more powerful?
Make it with more strongly curved surfaces
What is the power for a converging lens? (positive or negative)
Positive
Recall ray diagrams
(book)
Are electromagnetic waves transverse or longitudinal?
Transverse
Do EM waves travel at the same speed in a vacuum?
Yes
List the order of EM waves starting from those with the highest frequency/shortest wavelengths?
- Gamma rays
- X-rays
- Ultraviolet
- Visible light
- Infrared
- Microwaves
- Radiowaves
Give an example of how EM waves transfer energy to an observer
When you warm yourself by an electric heater, infrared waves transfer energy from the thermal energy store of the heater to the observers thermal energy store
The higher the frequency of the EM wave, the more ……… it transfers
(heat, energy, radiation)
Energy
Which part of the spectrum can our eyes detect?
Visible light
Is radio waves transmitted/absorbed or reflected through the body?
Radio waves are transmitted through the body without being absorbed
Are microwaves transmitted, absorbed or reflected and what is the effect of this?
Some wavelengths of microwaves can be absorbed, causing heating of the cells, which can be dangerous
Are IR and visible light transmitted, absorbed or reflected by the skin and what is the effect of this?
They are mostly reflected or absorbed by the skin causing some heating too. IR can cause burns if the skin gets too hot
is ultraviolet transmitted, absorbed or reflected by the skin and what is the effect of this?
It is absorbed by the skin. It’s dangerous due to its higher frequency . It is a type of ionising radiation, and when absorbed:
- it can cause damage to cells on the surface of the skin. This could lead to skin cancer.
- It could cause damage to the eyes. leading to eye conditions e.g. blindness
Are x-rays and gamma rays transmitted, absorbed or reflected by the skin and what is the effect of this?
They can be transmitted through the skin and be absorbed by deeper tissues. They can damage cells and cause cancer as:
- They are ionising,
- They have even higher frequencies, so they transfer more energy from a source to an observer
Do all objects continuously absorb and emit EM radiation over a range of wavelengths?
Yes
The distribution and intensity of these wavelengths depends on…
The objects temperature
As the temperature of an object increases, the intensity of every emitted wavelength…..
(increases, decreases)
Increases
Does intensity increase more rapidly (causing a lower peak wavelength) for shorter wavelengths or longer wavelengths?
For shorter wavelengths
When does an object heat up? (radiation)
If the average power that the object absorbs is more than the average power that it radiates.
When does an object cool down? (radiation)
If the average power that the object absorbs is less than the average power that it radiates
An object at a constant temperature radiates and absorbs …… average power
(more, less, the same)
The same
Explain the effect of radiation on the earths temperate during the day?
During the day, lots of radiation from the sun is absorbed and reflected by the atmosphere, clouds and surface. This causes an increase in local temperature
Explain the effect of radiation on the earths temperature at night?
At night, radiation emitted by the atmosphere, clouds and surface. This causes a decrease in local temperature
Overall, does the temperature of the earth change or does it stay fairly constant?
Stays fairly constant
How can radiation change the earths overall temperature?
If the atmosphere starts to absorb more radiation without emitting the same amount, the overall temp will rise until absorption and emission are equal again
Describe a practical that shows black surfaces are better emitters than white ones
- Wrap 4 identical test tubes with same material but different surface/colour e.g. black, white, glossy, matte e.t.c
- Boil water in kettle and fill each test tube with the same volume of water
- Use a thermometer to measure temp of water every minute. Seal test tubes with bungs between measurements
- Temp will decrease quicker for test tubes surrounded by good emitters. e.g. matte, dull, black surfaces
How could you create an alternating current using a radio wave?
- when transmitted radio waves, reach a receiver, the waves are absorbed
- The energy carried by the waves is transferred to the electrons in the receiver
- This energy causes the electrons to oscillate and generates an alternating current
- This current has the same frequency as the radio wave that generated it
What are radio waves used for? (long wave and short wave radio)
They are used for communications and broadcasting
- long wave radio can be received halfway around the world as long wavelengths bend around the curved surface of the earth
- short-wave radio signals can be received at long distances from transmitter as they’re reflected by the earths atmosphere
How are microwaves and radiowaves used in satellites
A signal from a transmitter is transmitted into space and picked up by the satellite receiver dish orbiting above the earth. The satellite transmits the signal back to Earth in a different direction, where its received by a satellite dish on the ground
What 3 things are microwaves used for?
- communications
- Satellites
- heating food
How are microwaves used to heat food?
Microwaves penetrate up to a few cm’s into food before being absorbed and transferring the energy they are carrying to the water molecules in the food, causing the water to heat up. The water transfers this energy to the rest of the molecules in the food by heating
What is thermal imaging and how can it be used?
Getting a camera to detect IR and turning it into an electrical signal that is displayed on a screen. It can be used by police to see suspects hiding or escaping in the dark
How can infrared sensors be used
They can be used in security systems, so that if IR is detected, an alarm sounds or a light turns on
Does absorbing IR make an object hotter or colder and what does this mean in terms of cooking?
hotter so it can be used to cook food
Give examples of things that use infrared to transfer information
- Sending files between mobile phones and laptops
- TV remotes
- Optical fibres
How does infrared allow optical fibres to carry data (from telephones/computers) over long distances?
They are carried as pulses of infrared radiation as a single wavelength, to prevent dispersion, which can otherwise cause data to be lost
Name a use of visible light
Photography
How is UV and visible light used in fluorescent lamps?
Ultraviolet radiation is absorbed by certain chemicals and visible light is emitted
What are some uses of fluorescent lights?
- Security pens to mark property and help identify stolen property
- Detecting forged bank notes and passports
What else can UV be used for?
Sterilising water by killing bacteria in the water and making it safe to drink
What are x-rays used for?
- To view internal structure of objects and materials as they are transmitted by flesh but absorbed by denser materials like bones
- Used in airport security scanners to detect hidden objects
What are gamma rays used for?
- for sterilisation - it kills microbes
- Tracers - to detect cancer
- Cancer treatment - targets cancer cells to kill them
What are radio waves produced by?
Oscillations in electrical circuits
