Grade 7 Chapter 17: Effects of Light

Introduction to Light

Light is a form of energy that enables us to see the world around us. It travels in straight lines and interacts with objects in various ways, leading to phenomena like reflection, refraction, and the formation of shadows. Understanding these effects helps us comprehend how light behaves and how it is used in different technologies.

17.1 Propagation of Light

Light Travels in a Straight Line

One of the fundamental properties of light is that it travels in straight lines. This is known as the rectilinear propagation of light.

• Evidence:
  • Shadow Formation: When an opaque object blocks the path of light, a dark area called a shadow is formed behind it. This happens because light cannot bend around the object.
  • Pin-hole Camera: A pin-hole camera forms an inverted image of an object. This is due to light rays traveling in straight lines from the object through the small hole to the screen.

17.2 Reflection of Light

What is Reflection?

Reflection is the bouncing back of light when it strikes a surface. The surface from which light bounces back is called a reflector.

• Examples: Mirrors, still water, polished surfaces.

Types of Reflection:

• Regular Reflection (Specular Reflection):
  • Occurs when light rays strike a smooth, polished surface (like a plane mirror).
  • Parallel incident rays reflect as parallel reflected rays.
  • Forms clear, sharp images.
• Irregular Reflection (Diffuse Reflection):
  • Occurs when light rays strike a rough or uneven surface (like a wall or paper).
  • Parallel incident rays reflect in different directions.
  • Does not form clear images; instead, it allows us to see objects from different angles.

Laws of Reflection:

Reflection follows two main laws:

1. First Law: The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
2. Second Law: The angle of incidence (angle between incident ray and normal) is equal to the angle of reflection (angle between reflected ray and normal). ($\angle i = \angle r$)

Images Formed by Plane Mirrors:

A plane mirror forms an image with the following characteristics:

• Virtual: The image cannot be obtained on a screen.
• Erect: The image is upright, not inverted.
• Laterally Inverted: The left side of the object appears as the right side in the image, and vice-versa.
• Same Size: The image is the same size as the object.
• Same Distance: The image is formed as far behind the mirror as the object is in front of it.

17.3 Refraction of Light

What is Refraction?

Refraction is the bending of light as it passes from one transparent medium to another transparent medium (e.g., from air to water, or water to glass). This bending occurs because light changes its speed as it moves from one medium to another.

• Examples:
  • A spoon appearing bent when placed in a glass of water.
  • The bottom of a swimming pool appearing shallower than it actually is.
  • Light passing through a lens.

Effects of Refraction:

• Apparent Depth: Objects in a denser medium (like water) appear shallower when viewed from a rarer medium (like air).
• Bending of Light Rays:
  • When light passes from a rarer medium (e.g., air) to a denser medium (e.g., water), it bends towards the normal.
  • When light passes from a denser medium (e.g., water) to a rarer medium (e.g., air), it bends away from the normal.

17.4 Lenses

A lens is a transparent optical device that focuses or disperses light rays by means of refraction. Lenses are typically made of glass or plastic and have at least one curved surface.

Types of Lenses:

• Convex Lens (Converging Lens):
  • Shape: Thicker in the middle and thinner at the edges.
  • Effect on Light: Converges (brings together) parallel rays of light to a single point called the principal focus.
  • Image Formation: Can form both real and virtual images, depending on the object's position. Real images are inverted, virtual images are erect.
  • Uses: Magnifying glasses, cameras, microscopes, telescopes, correcting farsightedness (hypermetropia).
• Concave Lens (Diverging Lens):
  • Shape: Thinner in the middle and thicker at the edges.
  • Effect on Light: Diverges (spreads out) parallel rays of light. The rays appear to originate from a single point (virtual focus) on the same side as the object.
  • Image Formation: Always forms a virtual, erect, and diminished (smaller) image.
  • Uses: Peepholes in doors, flashlights, correcting nearsightedness (myopia).

17.5 Dispersion of Light

What is Dispersion?

Dispersion of light is the phenomenon where white light splits into its constituent seven colors when it passes through a transparent medium like a prism or water droplets. This band of seven colors is called a spectrum.

• The Spectrum: The seven colors are Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
• Cause: Different colors of light travel at slightly different speeds in a medium other than vacuum, causing them to bend at different angles. Violet light bends the most, and red light bends the least.
• Natural Example: Rainbow formation. Rainbows are formed when sunlight passes through tiny water droplets in the atmosphere, which act like tiny prisms, dispersing the white light into its spectrum.