7: Light Hot objects emit heat (electromagnetic waves), and when red hot they also emits light. • Anything that gives off electromagnetic waves is called a source. All waves come from a source . Anything that gives off light waves is called a luminous source. Luminous objects can be seen because light from them enters the eye directly . Non-luminous objects are seen because light is reflected reflected from them and then enters the eye. Rays make beams. 3 types of beams, converging (coming together), diverging (moving apart) and parallel
Shadows are made because: a) Light travels in straight lines b) Opaque objects do not let light pass through c) The object blocks the light going to the screen. Light does not reach the wall and a shadow forms. Pointed light sources produce only an umbra. Extended light sources produce an umbra and penumbra . •
Reflecting Waves •
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called the normal. the normal. •
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All waves can be reflected . The best reflectors of light are smooth and shiny. In the lab, we use ray boxes to give us rays and beams of light. Experiments show that the angle of incidence equals the angle of reflection . (True for all waves) The line, at ninety degrees to the reflecting surface, where reflection occurs, is All measurements are made from
The image formed in a plane (flat) mirror has five characteristics. 1. It is laterally inverted (left-to-right). 2. It is the same size as the object 3. It is same the distance behind the mirror as the object is in front of the mirror 4. It is upright (the same way up as the object i.e. not inverted) 5. It is virtual (Cannot be focused on a screen) A periscope is used to see over walls. It consists of two plane mirrors set at 45° and parallel.
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Refraction (Bending of Light) Refraction occurs when light enters or leaves a transparent materials e.g. diamond, plastic and water. a) When light enters glass , it refracts towards the normal . (Angle gets smaller) b) When light leaves glass, it refracts away from the normal . (Angle gets •
bigger)
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c) Light that enters glass along the normal is not refracted . A spinning wheel painted with equal amounts of the seven colours appears white.
Dispersion and Coloured Light. White light splits into 7 colours when passed through a prism : red, orange, yellow, green, blue, indigo and violet. Roy G. Biv).
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All the colours in white light travels at the speed of light, but each colour is slowed down by a slightly different amount in glass, thus, each is refracted by a slightly different angle. Violet slow down the most so it refracts the most .
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We can make white light with only three colours, they are the primary colours red, green, and blue. Red + Blue + Green = White (This is true for lights not for paints.) Red + Blue = Magenta (like purple)
Blue + Green = Cyan (like turquoise)
Red + Green = Yellow
To work out the above remember: “ Well really, young girls can be moody.” (Can you draw it?) •
When light falls on an object it is reflected or absorbed. If it is reflected, the object gets its colour. If it is absorbed the object gets warmer as the light energy is converted into thermal energy.
Black objects absorb all three colours: red green and blue. No light is reflected into the eye. White objects do not absorb any of the 3 colours; all are reflected and combine to give white. •
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A daffodil appears yellow because it absorbed blue, but reflects red and green. R + G = Y White paper looks yellow under yellow (red and green) light because there is no blue to be reflected. Blue paper looks black in green light. The green light is absorbed and no light is reflected. •
If we view a yellow object in red light, it will appear red. This is because yellow is formed from the primary colours red and green. An object appears to be yellow because it only reflects red and green light, but if the green light is missing, then only red light can enter the eye so it appears red.
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Coloured filters can filter out light . A blue filter only lets blue pass through. (It filters out red and green light.) A red filter only lets red light through, and a green filter only lets green light through.
8: Sound Sounds are produced by something vibrating. Sound cannot travel through a vacuum. • (Know expt). We hear sounds because vibrations travel through the air to the ear and make the eardrum vibrate. Sound travels through the air by a series of compressions (air particle close together) and decompressions (air particles more spaced out). The speed of sound in air is about 330 m/s. Solids transmit sound vibrations quicker and better than liquids and gases . Sound waves are reflected and the angle of incidence equals the angle of reflection . Hard, smooth, solid surfaces are good sound reflectors . A reflected sound is called an echo. Soft irregular surfaces are bad sound reflectors and good sound absorbers . (E.g., music room) •
Amplitude determines the loudness of a Loud sounds have large sound. Soft amplitude. sounds have small amplitude. Frequency determines pitch. Things that vibrate quickly have a short wavelength and high frequency. High
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frequency gives a high pitch. Things that vibrate slowly have a long wavelength and a low frequency. Low frequency gives a low pitch. Humans can hear sounds between 20 and 20 000 hertz . Other animals can hear higher / lower frequencies. Frequency is a measure of the vibration per second or waves per second. Frequency is in hertz (Hz). •
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