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TNPSC Science Notes: 6th Standard: Physics: Light

TNPSC 6th Standard - General Science Notes 

TNPSC Notes for Group 4 Exam 2016

  • Galileo Galilei and Scientific Revolution. Galileo Galilei a great Italian scientist, he was a physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. 
  • Earlier, it was believed that the earth was stationary and it was in the centre of the solar system. 
  • The polish scientist Nicholaus Copernicus suggested that “The earth is not stationary. It spins on its own axis and revolves around the Sun”. This theory appealed to Galileo and he proved this concept by his experiments.
Telescope Invention (1609) 
  •  In 1609, Galileo invented the telescope, through which he saw the stars, planets and the moon. According to Galileo, the Sun is a star. All stars are like the Sun.
International Year of Astronomy - 2009
  • To commemorate the 400th anniversary of the telescope invention, the year 2009 was declared the “International Year of Astronomy”.
Sources of light 

The objects that emit light are called as sources of light. 
  • Sources of light are of two types natural and artificial. 
  • The sun is the primary and the natural source of light. 
  •  Some of the man-made objects also produce light. These are called artificial sources of light. 
  • Bodies that emit light on their own are called luminous bodies. 
  • Objects, that do not emit light on their own are called non-luminous bodies.
Natural sources of Light 
  • Sun
  • Fire flies
  • Jelly fish
Artificial sources of Light
  • Oil lamp
  • Decorative lamp
  • Chimney lamp
Though the moon looks bright, it is not a luminous body. It shines by getting light from the sun. The sunlight takes 8 minute and 20 second to reach the earth. We should not see the sun directly with naked eyes. It may affect our eyesight.

Transparent, Translucent and Opaque objects

Transparent objects

The objects which allow light to pass through them are called transparent objects. They appear blurred because water mixed with milk allows only some amount of light to pass through it.

Translucent objects.

The objects which allow the light to pass through them partially are called translucent objects. Air 
with dust particles, mist, ground glass, a sheet of paper smeared with oil are some more examples of translucent objects.

Opaque objects
  • The objects which do not allow light to pass through them are called opaque objects. Wooden door, plastic chair, brick are some more examples of opaque objects.
  • Shadow is always formed on the opposite side of the light source. Source of light, an opaque object and the shadow will always lie in a straight line. Shadow is cast since light travels in straight line. Solar and lunar eclipses occur because of this property of light. When the sun, the earth and the moon come in a straight line eclipses are formed.
Lunar Eclipse: (Eclipse of the moon)

When the earth comes between sun and the moon, lunar eclipse occurs. This happens on a full 
moon day.
  • The Sun - source of light 
  • The Earth - opaque object 
  • The Moon-screen
When the shadow of earth falls on the moon, the moon is hidden. This is called lunar eclipse. Solar Eclipse: (Eclipse of the sun)

When the moon comes between the sun and the earth, solar eclipse occurs. This happens on a new moon day. 
  • The Sun - source of light 
  • The Moon - opaque object 
The Earth-screen When the shadow of the moon falls on the earth, the sun is hidden.  This is called solar eclipse.

Reflection of light

When a beam of light falls on a smooth shining surface, it bounces back into the same medium (solid, liquid or gas). This phenomenon is called reflection.

Plane mirrors
  • Light rays from the light source which fall on our face are reflected. These reflected rays are reflected back again when they fall on a mirror. When these reflected rays from the mirror reach our eyes we are able This is called plane mirror.
When one side of the transparent glass is coated with a chemical substance, it becomes a plane mirror (opaque). All the highly polished paque objects show the property of reflection.

Important Questions & Answers

1. Who invented the telescope?
  • Galileo Galilei (1609)
2. When observed the "International Year of Astronomy"?
  • To commemorate the 400th anniversary of the telescope invention, the year 2009 was declared the “International Year of Astronomy”.
3. How much the time will take The sunlight to reach the Earth?
  • 8 minute and 20 second
Choose the correct answer

4. The name of the instrument invented by Galileo

a. Microscope b. Telescope c. Mirror

Answer: b. Telescope

5. Which is an artificial source of light?

a. Sun b. Fire fly c. Torch light

Answer: c. Torch light

6. An example for Non- luminous body?

a. Sun b. Candle c. Moon

Answer: a. Sun

7. Human body is _______________?

a. translucent b. opaque c. transparent

Answer: b. Opaque

8. The colour of the shadow of any coloured opaque object is?

a. white b. the same colour c. black

Answer: c. black

9. Reflected image can be clearly seen on a ______?

a. Polished surface b. Rough surface c. Shadow

Answer: a. Polished surface

Match the following (Matched)

10. Artificial light source - burning lamp

11. Non – luminous body - moon

12. Reflecting surface - disc

13. Translucent object - Turbid Water

14. Transparent object - spectacles

15. Opaque object - tree