What are rays?
Lines that show the direction in which waves travel.
What is the relation between wavefronts and rays?
Rays are always perpendicular to wavefronts.
What is the law of reflection?
The angle of reflection is equal to the angle of incidence.
What is refraction?
Bending of a wavefront at a boundary between two media, due to a change in wave velocity
How does the angle of incidence correlate with the angle of refraction?
As the angle of incidence increases, so does the angle of refraction (but not proportionally).
What is a critical angle?
A specific angle of incidence where the refracted light passes along the interface, and no light passes on the side where the less dense medium is.
What is total internal reflection?
It is when the angle of incidence becomes greater than the critical angle, then the light is completely reflected inside the glass.
What is the method of investigating internal reflection?
- Place a semicircular glass block on a large sheet of paper and draw around it.
- Remove the block. Using a ruler and pencil, find and mark the center of the straight side.
- Draw a normal line through this point, perpendicular to the straight side, then replace the block.
- The ray will bend away form the normal as it leaves the glass and some light is reflected by the straight surface of the glass block.
- Move the ray to increase the angle of incidence, which increases the angle of refraction. Continue increasing the angle of incidence until the reflected ray lines up with the flat surface of the block.
- Increase the angle of incidence again. The light is now reflected completely, no light escaping by refraction (total internal reflection). The angle of incidence at which total internal reflection begins is the critical angle.
What are optical fibres?
Narrow tubes of glass with a plastic coating that carry light from one end to the other. Light rays use total internal reflection to travel along the fibres.
What are the uses of optical fibres due to their properties?
- Medical: to transmit pictures of organs and arteries
- Industrial: to transmit pictures of the inside of complex machinery
- Communications: to transmit data over long distances without transmission loss
What is the method of investigating refraction?
- Place a piece of plain paper on a desk. Set up the power supply, ray box and single slit so that you can shine a single ray of light across the paper on your desk (take care as ray boxes can become very hot)
- Place rectangular glass block on paper and draw around it
- Shiny ray of light into block and use small crosses to mark where the rays of light go
- Take the block off the paper and use a ruler to join crosses to show path of light, extending the lines so they meet the outline of the block. Also join the points where the light entered and left the block to show where it travelled inside
- Measure angles of incidence and refraction where light entered the block and measure angles where it left the block
- Repeat steps 3-5 with ray entering block at different angles
- Move ray box so light ray reaches interface at right angles and note what happens to the light as it enters and leaves
What is a luminous object?
Objects that we can see as they emit light.
What is a non-luminous object?
Objects that we can see as they reflect light.
What is the visible colour spectrum (increasing wavelength)?
- Violet (400 nm)
- Indigo (425 nm)
- Blue (470 nm)
- Green (550 nm)
- Yellow (600 nm)
- Orange (630 nm)
- Red (665 nm)
What is a lens?
A piece of transparent material shaped to refract light.
What is the power of a lens?
How much it bends light that passes through. The more it is curved the greater the bending of light.
What can the different characteristics be of images formed by a lens?
- Upright or inverted
- Magnified or diminished
- Real or virtual
Check diagram on how to draw.
What does the image appear to be if the object is placed more than twice the focal length from the convex lens and what is an example of this?
- Inverted
- Diminished
- Real
E.g. Camera, human eye
Check diagram on how to draw.
What does the image appear to be if the object is placed between one and two focal lengths from the convex lens?
- Inverted
- Magnified
- Real
E.g. Projector
Check diagram on how to draw.
What does the image appear to be if the object is placed at a distance less than the focal length from the convex lens?
- Upright
- Magnified
- Virtual
E.g. Magnifying glasses
Check diagram on how to draw.
What does the image appear to be if the object is placed behind a concave lens?
- Upright
- Diminished
- Virtual
E.g. Peep hole lenses
Check diagram on how to draw.
What are the characteristics of E/M waves?
- Transverse waves
- Can propagate through any medium and vacuum
- In vacuum they cover all distance at the same rate
- Higher frequency waves have more energy
What are the different types of E/M waves?
- Radio waves
- Microwaves
- Infrared
- Visible light
- Ultraviolet (UV)
- X-rays
- Gamma rays
How were infrared waves discovered?
William Herschel used a prism to split sunlight into a spectrum and put thermometers in each colour in turn. He also measured the temperature just beyond the red end of the spectrum. He noticed that temperature increased and realised there must be another type of light which we cannot see in this region.
