|| Just what you need to know !
Light is an invisible energy, which causes the sensation of vision in us through our eyes.
Luminous bodies are those that radiate light.
For example, the sun, stars and fire are luminous bodies.
Light from the sun takes approximately eight minutes to reach the earth's surface.
Non-luminous bodies are those that do not radiate their own light and they become visible only when they reflect another source of light into our eyes.
For example, the moon and the planets are non-luminous bodies.
Light always travels in straight lines.
A light ray is the straight line path along which light energy travels in a given direction.
A parallel beam is a collection of light rays in which the rays always remain the same distance apart.
A convergent beam is a collection of light rays in which the rays slowly converge (come close together and finally meet at a point) as the beam progresses.
A divergent beam is a collection of light rays in which the rays slowly diverge (go far apart starting from a point) as the beam progresses.
Optical medium is anything through which light energy can pass partially or totally.
Transparent medium is one which allows light energy to pass through almost completely.
Hence, all objects are visible and can be clearly seen through transparent media such as water and clear glass.
Translucent medium is one which allows light energy to pass through partially (but not completely).
Hence, objects are visible but they cannot be clearly seen through translucent media such as frosted glass and thin tissue paper.
Opaque body is one which does not allow light energy to pass through at all.
Hence, objects cannot be seen through opaque bodies such as wood and metal.
Shadow is a dark patch formed behind an opaque body when it is placed in the path of light.
It consists of two regions, the umbra and the penumbra.
Umbra is the region of total darkness where no light enters.
Penumbra is the region of partial darkness which surrounds the umbra.
Eclipse is the shadow cast by the heavenly bodies on each other.
Solar eclipse is caused when the moon is between the sun and the earth.
The moon occasionally becomes aligned between the sun and the earth.
Viewed from parts of the earth, the moon covers the sun partially or totally.
This blocks the light and causes a brief period of darkness known as a (partial or total) solar eclipse.
When the moon occasionally becomes precisely aligned between the sun and the earth, it covers the center of the sun perfectly.
Only an annular ring of sunlight appears around the moon's shadow.
This type of solar eclipse is called an annular eclipse.
Lunar eclipse is caused when the earth is between the sun and the moon.
The earth at times becomes aligned between the sun and the moon.
When this happens, the earth blocks light from the sun and cast its shadow across the moon.
Viewed from the earth, this shadow crossing the moon is known as a lunar eclipse.
The lunar eclipse occurs more frequently and lasts for longer time periods than the solar eclipse.
Illuminance of a surface equals the power (luminous intensity) of the light source divided by the square of the distance.
∴ Illuminance of a surface = Power (luminous intensity) of light source / Distance2
Usually, illuminance is measured in lux and the power (luminous intensity) of the light source is measured in candelas (abbreviated as cd), while the distance is measured in meters.
The photometer is the instrument used to compare the luminous intensities (powers) of two light sources.
Physics Quiz on Light.