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Space Telescope mirrors Space telescope mirrors Scientists of Ball Aerospace in collaboration with NASA found new ways to build a telescopic mirror which is very light and yet very strong. The hexagonal shape allows a segmented mirror with high filling factor (the segments fit together without gaps). A roughly circular overall mirror shape is desired because that focuses the light into the most compact region on the detectors. What does a mirror do? What are the other types of mirrors? Lets discuss these in detail.

Learning objectives

After completing the topic, the student will be able to:

  • Discuss and explore the phenomenon of reflection of light based on various observations in daily life.
  • Understand the concept of refractive Index by observing simple experiments related to daily life scenarios.
  • Investigate and distinguish between reflection and refraction of light using simple observations of daily life.
  • Examine the laws of refraction and incorporate them to understand the dual nature of light.
  • Examine the image formation in plane mirrors for different cases.
  • Explore the types of reflections formed in convex and concave mirrors: expand its use in various applications like telescopes, side mirrors of vehicles.
  • Analyze the importance of diffused reflection.
Reflection of light from a mirror When light rays traveling is a medium reaches the boundary of other medium, they return back to the first medium. This phenomenon of turning back of light into the same medium after striking the boundary of other medium is called Reflection of Light.
Reflection of light

We have learnt in the last chapter that Newton proposed the corpuscular theory that treats light as being composed of tiny particles. We use this theory to describe reflection.

When light falls on the surface of an object, it may be absorbed, transmitted or reflected. If an object absorbs all the light which falls on it, it will appear black. For example, a black board appears black because it absorbs all the visible frequencies.

If an object transmits the light, that is, if the object allows the light to pass through itself, the object is said to be transparent. For example, an ordinary glass sheet transmits most of the light that falls on it and hence appears transparent. Most of the objects reflect some of the light which falls on their surface.

When a polished block of silver metal is illuminated with sunlight or any source of light, electrons in the atoms of the silver vibrate more energetically in response to the oscillating electric fields of the illuminating light. The energized electrons re-emit the light by which we see the dazzling. The process of sending back the light rays which fall on the surface of an object, is called reflection of light.

Absorption of light When light rays enters from medium, only a part of it is reflected back, the remaining part of the light is absorbed by the medium or transmitted through it.
Reflection, absorption and transmission

When light falls on a surface three things may happen:
i) A fraction 'r' will be reflected.
ii) A fraction 'a' will be absorbed.
iii) A fraction 't' will be transmitted.
For any surface these fractions must add up to 1.
Therefore :
r + a + t = 1

Magnesium oxide will reflect some 98% of the incident light. Silver is also a very good reflector over a fairly limited range of wavelengths but does tarnish with age. Aluminum although not quite so reflective as silver gives a better reflection over a wider range of wavelengths. The absorption of light as it passes through a material is very important especially in the area of fibre optic communication systems.

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