Quick Revision Notes: Class 7 Science

Chapter: Light (प्रकाश)

### Concept 1: Propagation of Light

• Light: A form of energy that enables us to see objects around us. It always travels in a straight line.
• Rectilinear Propagation of Light (प्रकाश का सीधी रेखा में गमन): The property of light travelling along a straight line is called rectilinear propagation.
• Beam of Light: A collection of many rays of light.
  • Parallel beam: Rays are parallel to each other.
  • Convergent beam: Rays meet at a point.
  • Divergent beam: Rays spread out from a point.

### Concept 2: Reflection of Light (प्रकाश का परावर्तन)

• Reflection: The bouncing back of light into the same medium after striking a polished, shiny surface (like a plane mirror).
• Important Terms:
  • Incident Ray (आपتित किरण): The ray of light that falls on the reflecting surface.
  • Reflected Ray (परावर्तित किरण): The ray of light that bounces back from the surface.
  • Normal (अभिलंब): A line perpendicular (at 90 degrees) to the reflecting surface at the point of incidence.
  • Angle of Incidence (i): The angle between the incident ray and the normal.
  • Angle of Reflection (r): The angle between the reflected ray and the normal.

Laws of Reflection (परावर्तन के नियम):

1. The angle of incidence is always equal to the angle of reflection. (Formula: ∠i = ∠r)
2. The incident ray, the normal to the surface at the point of incidence, and the reflected ray all lie in the same plane.

### Concept 3: Types of Images Formed by Mirrors

• Image: The optical appearance of an object formed by light.
• Real Image (वास्तविक प्रतिबिंब):
  • Can be obtained on a screen.
  • Always inverted.
• Virtual Image (आभासी प्रतिबिंब):
  • Cannot be obtained on a screen.
  • Always erect (upright).

Characteristics of Image formed by a Plane Mirror:

1. Always virtual and erect.
2. Equal in size to the object.
3. At the same distance behind the mirror as the object is in front of it.
4. Laterally inverted (Left appears right, and right appears left).

### Concept 4: Spherical Mirrors (गोलीय दर्पण)

Mirrors whose reflecting surfaces are curved are called spherical mirrors.

1. Concave Mirror (अवत्त दर्पण):
   • Reflecting surface is curved inwards.
   • Can form real and inverted images (as well as virtual and erect when the object is very close).
   • Used by dentists, in torches, headlights of cars.
2. Convex Mirror (उत्तल दर्पण):
   • Reflecting surface is curved outwards.
   • Always forms virtual, erect, and diminished (smaller) images.
   • Used as rear-view mirrors in vehicles because they provide a wider field of view.

### Concept 5: Lenses (लेंस)

Transparent curved materials used to refract light.

1. Convex Lens (उत्तल लेंस):
   • Thicker in the middle than at the edges.
   • Also called Converging Lens (converges light rays).
   • Forms real and inverted images (and virtual when the object is very close, like a magnifying glass).
2. Concave Lens (अवत्त लेंस):
   • Thinner in the middle than at the edges.
   • Also called Diverging Lens (spreads out light rays).
   • Always forms virtual, erect, and diminished images.

### Concept 6: Sunlight - White or Colored?

• Dispersion of Light (प्रकाश का विक्षेपण): White light is composed of seven colors (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red).
• Rainbow: A natural phenomenon showing the dispersion of sunlight through tiny water droplets suspended in the air after rain.
• Newton’s Disc: A cardboard disc painted with the seven rainbow colors which, when rotated fast, appears white, proving that sunlight consists of seven colors.