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, a wave is reflected from a smooth, flat surface and all light incident at the same angle all exit at the same angle and 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 how colour of light is related to transmission
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 spreads light outwards. Light appears to have come fro the focal point.
In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows 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 outward 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.
