📖 Chapter Overview
Light is a form of energy that enables us to see objects. This chapter covers the nature and behavior of light — its straight-line propagation, reflection, refraction, dispersion, and the human eye. It also explains phenomena like shadows, eclipses, lenses, and applications such as mirrors and optical instruments.
Rectilinear propagation
Reflection — laws
Refraction — bending of light
Lens, dispersion, and eye defects
Quick tip: Draw rays with arrows to show direction; label angle of incidence
i and angle of reflection r.🔑 Key Points
- Light travels in straight lines — demonstrated by shadow formation and pinhole camera.
- Reflection: Angle of incidence = Angle of reflection; reflected ray, incident ray and normal lie in same plane.
- Plane mirror forms a virtual, erect image of the same size as the object; image distance = object distance.
- Refraction: Bending of light when it passes from one medium to another; depends on refractive index.
- Light bends towards normal when entering denser medium (air → glass) and away when entering rarer medium.
- Lens: Convex (converging) and concave (diverging) lenses; uses — magnifying glass, spectacles.
- Dispersion: White light splits into colors on passing through a prism due to different speeds.
- Human eye: cornea, lens, retina, optic nerve; vision defects include myopia and hypermetropia corrected by appropriate lenses.
📐 Important Relations & Concepts
| Topic | Key Idea / Relation |
|---|---|
| Rectilinear propagation | Light travels in straight lines in a uniform medium. |
| Reflection (plane mirror) | Image is virtual, upright and of same size; u = -v convention in higher classes. |
| Refraction | Snell's law (idea level): bending depends on refractive indices; refractive index = speed in vacuum / speed in medium (introduced in higher classes). |
| Lens | Convex focuses parallel rays to principal focus; concave diverges rays. |
Pinhole camera: inverted image on screen due to straight-line travel of light through a small aperture.
🧪 Applications & Examples
| Application | Uses/Principle |
|---|---|
| Periscope | Uses two plane mirrors to view over obstacles (law of reflection). |
| Magnifying glass | Convex lens produces magnified virtual image when object is within focal length. |
| Spectacles | Correct vision defects using concave (for myopia) or convex (for hypermetropia) lenses. |
| Prism | Splits white light into colors (dispersion). |
📚 30 Important Questions & Answers
Q1. What is light?
Light is a form of energy that enables us to see objects. It travels in straight lines in a uniform medium.
Q2. Give an experiment that shows light travels in straight lines.
Pinhole camera or shining a torch through aligned holes in cardboard — light creates straight-line shadows.
Q3. State the laws of reflection.
1) Angle of incidence equals angle of reflection. 2) Incident ray, reflected ray and normal lie in the same plane.
Q4. What type of image is formed by a plane mirror?
A virtual, erect image of the same size as the object; it appears to be as far behind the mirror as the object is in front.
Q5. What is refraction of light?
Refraction is the bending of light when it passes from one medium to another due to change in speed.
Q6. Give an example of refraction in daily life.
A straw appearing bent in a glass of water; apparent depth of a swimming pool.
Q7. Why does light bend towards the normal when entering a denser medium?
Because the speed of light decreases in denser media, causing the ray to change direction towards the normal.
Q8. What is dispersion of light?
Dispersion is splitting of white light into its constituent colors due to different degrees of refraction for different wavelengths.
Q9. Which color bends the most in a prism?
Violet bends the most; red bends the least.
Q10. What is a lens?
A lens is a transparent material (usually glass) with curved surfaces that refract light to form images.
Q11. Difference between convex and concave lens?
Convex lens converges parallel rays to a focus; concave lens diverges them as if from a virtual focus.
Q12. How does a magnifying glass work?
A convex lens produces a magnified virtual image when the object is placed within its focal length.
Q13. What causes a rainbow?
Refraction and dispersion of sunlight by water droplets in the atmosphere, followed by internal reflection and dispersion.
Q14. What is the function of the cornea in the human eye?
Cornea refracts light entering the eye and contributes to focusing on the retina.
Q15. What is the retina?
Retina is the light-sensitive layer at the back of the eye where images are formed and converted into nerve signals.
Q16. What is myopia and how is it corrected?
Myopia (short-sightedness) is when distant objects are blurred; corrected using concave (diverging) lenses.
Q17. What is hypermetropia and its correction?
Hypermetropia (long-sightedness) is when near objects are blurred; corrected with convex (converging) lenses.
Q18. What is a shadow?
A dark area formed when an opaque object blocks light from a source.
Q19. What is an eclipse?
An eclipse occurs when an astronomical body (Moon/Earth) blocks light from another body, forming shadows (solar or lunar eclipse).
Q20. What is a periscope and its use?
A periscope uses two plane mirrors to see over or around obstacles; used in submarines.
Q21. Explain why objects look bent in water.
Due to refraction: light from the object bends at the water surface, making the object appear shifted.
Q22. What is total internal reflection (idea level)?
When light tries to go from denser to rarer medium beyond a critical angle, it reflects back into the denser medium; used in fiber optics (introduced higher classes).
Q23. How do we see colours?
Objects reflect certain wavelengths of light; those wavelengths reach our eyes and are interpreted as colours by the brain.
Q24. Why does a convex mirror form a virtual diminished image?
Convex mirror causes parallel rays to diverge; the image appears behind the mirror and is smaller than the object.
Q25. Give one use of convex mirrors.
Used as rear-view mirrors in vehicles for a wider field of view.
Q26. If an object is placed at twice the focal length of a convex lens, what type of image will be formed? (Idea level)
A real, inverted image of the same size as the object, formed at twice the focal length on the other side.
Q27. Why do stars appear twinkling?
Atmospheric refraction causes starlight to bend slightly and vary, making stars appear to twinkle.
Q28. What is the principal focus of a lens?
The point where parallel rays of light either converge (convex) or appear to diverge from (concave) after refraction.
Q29. How does a camera lens form images?
A convex lens focuses light from an object onto a film or sensor to form a real image; focal length and aperture control image properties.
Q30. Why are optical instruments important?
They extend human vision and study of light (microscopes, telescopes, spectacles), helping in science and daily life.