Light: Reflection and Refraction
Light is a form of energy that creates the sensation of vision. It travels in straight lines (Rectilinear Propagation). But what happens when it hits a wall? Or when it enters a glass of water? This is where Physics gets interesting.
For Class 10 students, this chapter is the foundation of Optics. We will break this down into three major conceptual pillars: Reflection, Refraction, and Lenses.
Part 1: Reflection of Light
Reflection is the bouncing back of light rays when they strike a polished surface. The most common example is a mirror.
1. Laws of Reflection
Whether the surface is flat (plane mirror) or curved (spherical mirror), these two laws are universal:
- First Law: The angle of incidence (∠i) is always equal to the angle of reflection (∠r).
- Second Law: The incident ray, the reflected ray, and the normal (the perpendicular line at the point of incidence) all lie in the same plane.
(Opens in a new tab)
2. Real vs. Virtual Images
Before we look at mirrors, you must understand the two types of images formed:
| Real Image | Virtual Image |
|---|---|
| Formed when light rays actually meet. | Formed when light rays appear to meet (when extended backwards). |
| Can be obtained on a screen (like a cinema projector). | Cannot be obtained on a screen (like your face in a mirror). |
| Always Inverted (Upside down). | Always Erect (Upright). |
3. Spherical Mirrors
Spherical mirrors are parts of a hollow sphere. There are two types:
- Concave Mirror (Converging): The reflecting surface is curved inwards. It collects light to a single point called the Focus.
Application: Used by dentists to see large images of teeth, shaving mirrors, and solar furnaces. - Convex Mirror (Diverging): The reflecting surface is curved outwards. It spreads light out.
Application: Rear-view mirrors in cars because they give a much wider field of view than plane mirrors.
4. The Mirror Formula
To find the position of the image mathematically, we use the mirror formula:
Magnification (m): This tells us how much larger or smaller the image is compared to the object.
- Light always comes from the Left.
- u (Object distance): Always Negative (-).
- f (Concave): Negative (-).
- f (Convex): Positive (+).
Part 2: Refraction of Light
Refraction is the bending of light when it travels from one transparent medium to another (e.g., Air to Glass). This happens because light travels at different speeds in different media.
1. The Rules of Bending
- Rare to Dense (Air to Water): Speed slows down → Light bends TOWARDS the normal.
- Dense to Rare (Glass to Air): Speed speeds up → Light bends AWAY FROM the normal.
2. Refraction through a Rectangular Glass Slab
When light passes through a glass slab, it refracts twice (Air→Glass, then Glass→Air). The final emergent ray is parallel to the original incident ray, but it is shifted sideways. This shift is called Lateral Displacement.
📷 Click here to View Diagram: Refraction through Glass Slab3. Refractive Index (n)
The refractive index is a measure of how much a medium slows down light. It is derived from Snell's Law.
Absolute Refractive Index:
Example: The refractive index of diamond is 2.42. This means light travels 2.42 times slower in diamond than in vacuum!
Part 3: Spherical Lenses
Mirrors reflect light; Lenses refract light. A lens is a piece of transparent material bound by at least one curved surface.
1. Convex vs. Concave Lens
- Convex Lens (Converging): Thicker at the center, thinner at the edges. It brings parallel rays to a Focus. It is used in cameras and to correct Hypermetropia (farsightedness).
- Concave Lens (Diverging): Thinner at the center, thicker at the edges. It spreads light out. It is used in peepholes and to correct Myopia (nearsightedness).
2. The Lens Formula
Be careful! The sign changes here compared to the mirror formula.
Magnification (m):
3. Power of a Lens
The power of a lens is its ability to converge or diverge light rays. It is the reciprocal of the focal length.
The SI unit of Power is Dioptre (D).
- Power of Convex Lens = Positive (+)
- Power of Concave Lens = Negative (-)
Summary Cheat Sheet for Numericals
Use this table to quickly solve any board exam problem:
| Quantity | Concave Mirror | Convex Mirror | Convex Lens | Concave Lens |
|---|---|---|---|---|
| u (Object) | Always (-) | Always (-) | Always (-) | Always (-) |
| f (Focal Length) | (-) | (+) | (+) | (-) |
| v (Real Image) | (-) | N/A | (+) | N/A |
| v (Virtual Image) | (+) (Behind mirror) | (+) (Behind mirror) | (-) (Same side) | (-) (Same side) |
Comments
Post a Comment