Tag: opque

Topic tree – Level STD 7 – IGCSE Science

Drawings by Himesh, STD 07

1. How light travels
2. How shadows form
3. How reflections form
4. How light bends
5. The spectrum of white light
6. Coloured light

HOW LIGHT TRAVELS

Sources of light	Object that emits its own light is called light source. e.g. flames, Sun, bulb
Straight lines	Light travels in straight line. e.g. Sun rays
Luminous and non-luminous	An object that acts as source of light – luminous An object that does not act as source of light, it reflects light to our eyes – non-luminous

HOW SHADOWS FORM

• Types of material
  1. Transparent – light passes straight through the object
  2. Opaque – The light is absorbed by the object, the light may bounce off the object, the material gets slightly warmer.
  3. Translucent – semitransparent – partly transparent and partly opaque
• Forming a shadow
  1. Shadow is formed when an opaque object blocks the light.
  2. Because, the light travels in straight line.
  3. Shadows change during the day as the Sun moves across the sky.
  4. Their direction and size change.

• Rays of light
  1. A ray is a straight line which shows the path of light.
  2. A shadow forms when an opaque object blocks light.

HOW REFLECTIONS FORM

• Bouncing of light
  1. A mirror gives a clear image of the object kept in front of it.
  2. Mirrors reflect light. It gives clear image of the object kept in front of it. Because, it has flat and smooth surface. Rays of light bounce off a mirror without being scattered. All reflected rays are in the same direction.

• White paper also reflects light. But, it does not give clear image. Because, it has rough surface. When the light falls on it, they are scattered in all directions.

• Predicting the path of light
  1. The law of reflection – angle of incidence = angle of reflection
  2. The mirror is represented by a straight line shaded on one side which indicates the non-reflecting surface.

HOW LIGHT BENDS

• Transparent materials
  1. Water and glass are transparent materials.
  2. Light can pass through them, but the direction of light ray is changed when it enters or exits a transparent material.
  3. Change of direction of light when it travels from one medium to another is called refraction.
• Rays changing direction
  1. A light ray bends towards the normal when it passes from air into a transparent material.
  2. The light ray bends away from the normal when it passes from a transparent material into air.

• The hidden coin
  1. A coin is placed in the bottom of a can.
  2. It is visible from the top.
  3. But, when we mover our head near the rim of the can, the coin disappears.
  4. When we pour water and again check the coin, it appears again!
  5. This feature is explained by bending of light when it travels through another medium, here it is the water poured.

SPECTRUM OF WHITE LIGHT

• Splitting light
  1. White light is made up of all colours of the spectrum from red to violet.
  2. VIBGYOR – Violet, Indigo, Blue, Green, Yellow, Organe and Red
  3. Splitting of white light into separate colours is called dispersion.
  4. E.g. Rainbow colours, spectrum obtained from prism
  5. Prism is a triangular glass block.
• Dispersion