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Std 6 General Science · Chapter 7

7. A Journey of Light

Complete solutions to the Exercises, with a fully labelled reflection diagram. Maharashtra State Board (Balbharati), Grade Six General Science.

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About the chapter

We see an object only when light from it reaches our eyes. This chapter tells us about the sources of light and how light travels. We learn about the pinhole camera. We also learn the laws of reflection.

Key points

  • The Sun is the main natural source of light for the Earth.
  • Objects that give their own light are self-luminous. Others are non-luminous.
  • Light travels in a straight line.
  • A pinhole camera gives an inverted image.
  • The angle of incidence is always equal to the angle of reflection.
  • A smooth surface gives regular reflection. A rough surface gives irregular reflection.

Words to remember

  • incident ray – the ray of light that falls on a surface
  • reflected ray – the ray that bounces back from the surface
  • normal – the line perpendicular to the surface at the point of incidence
  • point of incidence – the point where the incident ray strikes the surface
  • reflection – the bouncing back of light from a surface

Exercises

1Fill in the blanks.

(1) The Sun is the major natural source of light for the Earth.

(2) When light falls on opaque objects, the reflection of light occurs.

(3) The functioning of a pinhole camera depends on the straight-line propagation of light.

(4) A beam of light coming through a window is visible due to the dust particles floating in the air.

(5) Reflection of light from still water is of the regular type.

2Choose the correct option.

(1) If the angle of incidence is 30°, then the angle of reflection is …… degrees.

  • (a) 0
  • (b) 60
  • (c) 30  — correct
  • (d) 90

(2) A clear image is seen in a plane mirror because ……

  • (a) regular reflection of light occurs.  — correct
  • (b) the surface of the mirror is rough.
  • (c) the mirror is transparent.
  • (d) irregular reflection of light occurs from the mirror.

(3) We can see the moon because—

  • (a) The moon is self-luminous.
  • (b) The moon is a satellite of the earth.
  • (c) Sunlight is reflected from the surface of the moon.  — correct
  • (d) The moon is close to the earth.

(4) When are the angle of incidence and the angle of reflection equal during reflection of light?

  • (a) Always  — correct
  • (b) Sometimes
  • (c) Under specific conditions
  • (d) Never
3The moon is approximately four lakh kilometers away from the earth. The moon is visible from earth, but any sounds produced there cannot be heard on earth. Why is this so?
  1. Sound needs a medium like air to travel.
  2. There is no air between the moon and the earth.
  3. So sound from the moon cannot reach us.
  4. Light does not need a medium, so we can see the moon.
4Identify whether the following statements are true or false. Rewrite the false statements after correcting them.

(1) The incident ray is called the normal.

False. The line perpendicular to the surface at the point of incidence is called the normal.

(2) We can see an object placed in front of us through a bent pipe.

False. We can see an object through a straight pipe only.

(3) Objects are clearly visible to us in a dark room.

False. Objects are not visible in a dark room.

(4) The image obtained in a pinhole camera is inverted.

True.

5Who will be able to see the candle flame?

Rekha will see the candle flame.

RekhaSahilJohnRuby✓✗✗✗Candlestraight line of sight

Only Rekha’s pipe is straight.

Her pipe is straight. Light travels in a straight line. So light from the flame reaches her eyes. The other pipes are bent. So light cannot reach their eyes.

6Label the diagram.
NormalIncident rayReflected ray∠i∠rAngle of incidenceAngle of reflectionOPoint of incidence (O)Plane mirror (reflecting surface)Torch

The labelled diagram.

  1. Incident ray
  2. Normal
  3. Reflected ray
  4. Point of incidence (O)
  5. Angle of incidence (∠i)
  6. Angle of reflection (∠r)
  7. Plane mirror
7An image of the moon is visible on calm water. If there are ripples on the water, does the image remain the same? What is the difference and what is the reason for it?

No, the image does not remain the same.

  1. On calm water, a clear image is seen because of regular reflection.
  2. On rippled water, the image breaks and shakes.
  3. Ripples make the surface uneven.
  4. So light is reflected in different directions.
  5. This is irregular reflection.
8Classify the following into two groups: Self-luminous and Non-luminous. (An aeroplane visible in the sky, a book, the sun, gold, fire, the moon, the flame of an oil lamp, the oil lamp itself, the oil in the oil lamp.)

Self-luminous: The sun, fire, the flame of an oil lamp

Non-luminous: An aeroplane visible in the sky, a book, gold, the moon, the oil lamp itself, the oil in the oil lamp

← All Std 6 Science chapters 8. Beyond the Sky →