Light Class 7 Science Notes - Chapter 15

Does light travel in straight line?

  • The path of light gets blocked when an object covers the source of light. This is illustrated by trying to see the flame of a candle using a straight pipe and then with a bent pipe. This proves that light travels in a straight line.

Based on their interaction with light, objects can be classified as:

  • Transparent objects allow light to pass through them completely.E.g: glass
  • Translucent objects allow light to pass through them partially. E.g: Butter paper
  • Opaque objects do not allow any light to pass through them. E.g: table, book, etc.

The letters ABCD as seen through a
(A) Opaque object
(B) Translucent object
(C) Transparent object

Reflection

When light is incident on a certain surface, it either gets reflected or bounces back. Such surfaces are really well polished and act like a mirror. The phenomenon of light bouncing off surfaces is called reflection.

Laws of reflection

  • The ray of light that strikes a reflecting surface is called as the incident ray and the ray that gets reflected back is called the reflected ray
  • The imaginary line that is perpendicular to the reflecting surface at the point of incidence is called the normal
  • Angle of incidence ∠i is the angle between the incident ray and the normal. Angle of reflection ∠r is the angle between the reflected ray and the normal.
  • Law of Reflection states that the angle of incidence ∠i is always equal to the Angle of reflection ∠r (∠i=∠r). The angle of incidence, angle of reflection and the normal all lie on the same plane

Images

  • Rays of light from an object after reflection by a mirror tend to converge or appear to diverge from a certain point in front of the mirror. At this point, the reproduction of the object is called as an image.
  • These images are classified as real or virtual images.
    • A real image is formed by the actual convergence of light rays after reflection. Real images can be viewed on a screen.
    • A virtual image is the apparent convergence of diverging light rays after reflection. Virtual images cannot be viewed on a screen.
  • The image formed by a plane mirror is erect, virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it.

Range of visibility (plane mirrors)

  • An observer can only see a reflection on a plane mirror if the observer is within its range of visibility.
  • If an observer wants to see his entire image on a plane mirror, the mirror must be at least half the height of the observer.

Lateral inversion

  • An image formed by a plane mirror undergoes lateral inversion, i.e the right side of the object appears as the left side in the image.
  • That is why the word “AMBULANCE” is written backwards on ambulance vans so that it appears in the correct order when viewed on a mirror.

Spherical mirrors

  • A spherical mirror (or curved mirror) is a mirror which has the shape of a piece cut out of a spherical surface. They are of 2 types: Concave and Convex.


  • Concave mirror: If the outer surface of the curved mirror is painted and its inner surface is a reflecting surface, then this type of spherical mirror is a concave mirror.
  • Convex mirror:If the inner surface of the curved mirror is painted and its outer surface is a reflecting surface, then this type of spherical mirror is a concave mirror.

Image formation by spherical mirrors

  • Concave mirrors form a real, inverted and magnified image. When the object is moved really close to the mirror, the image formed is erect and virtual. Concave mirrors are used by doctors and dentists for enlarged images of our ears, eyes, teeth etc.
  • Convex mirrors form erect and virtual images that are diminished, meaning smaller than the object size. That is why convex mirrors are used in vehicles as it gives the driver images spread over a large area.

Lenses

Refraction

  • The bending of the path of light when it moves from one medium to another is known as refraction.
  • The object we see after refraction is called the apparent object.
  • There exists some displacement between the apparent and actual object.

Image formation by lenses

  • Convex lens forms a real, inverted and diminished image. When the object is placed very close the lens the image formed is erect, virtual and magnified. They are also called as converging lens. This type of lens is also used as a magnifying glass.
  • Concave lens always forms a virtual, erect and diminished image. They are also called as diverging lens. They are used as camera lenses and binoculars.

 

Dispersion of white light through prism

  • Sunlight may appear as white but in fact, is made up of seven component colours. This can be observed when a beam of sunlight is passed through a triangular prism or when light falls on a compact disk (CD).
  • The seven colours that appear are violet, indigo, blue, green, yellow, orange and red. These are also the colours of a rainbow we see after a rainfall.
  • The splitting of white light into its component colours is called dispersion.

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