Light allows us to see objects around us.
Without light, vision is impossible.
Optics is the branch of physics that deals with the behaviour and properties of light.
It explains how light travels, how it reflects and bends, how images are formed, and how the human eye works.
This chapter connects physics with daily experiences like:
- Seeing our reflection in a mirror
- Wearing spectacles
- Observing objects through lenses
- Understanding vision defects
Table of Contents
Rectilinear Propagation of Light
Light travels in a straight line in a uniform medium. This property is called rectilinear propagation of light.
Evidence of Rectilinear Propagation
- Formation of shadows
- Solar and lunar eclipses
- Sharp edges of shadows
Example:
When an opaque object is placed in the path of light, a shadow is formed behind it because light cannot bend around the object.
Pin Hole Camera
A pin hole camera works on the principle of rectilinear propagation of light.
- Light travels in straight lines
- Image formed is inverted and real
This proves that light does not curve while travelling in a uniform medium.
Reflection of Light
Reflection is the bouncing back of light when it strikes a smooth surface.
Laws of Reflection
There are two laws of reflection:
- The incident ray, reflected ray, and normal lie in the same plane.
- Angle of incidence equals angle of reflection.
These laws apply to all reflecting surfaces.
Regular and Irregular Reflection
- Regular reflection occurs from smooth surfaces and produces clear images.
- Irregular reflection occurs from rough surfaces and does not produce images.
Spherical Mirrors
Spherical mirrors are mirrors whose reflecting surface is part of a sphere.
There are two types:
- Concave mirror
- Convex mirror
Concave Mirror
A concave mirror has its reflecting surface curved inward.
Properties:
- Can form real or virtual images
- Used to focus light
Uses:
- Shaving mirrors
- Headlights of vehicles
- Solar furnaces
Image formed depends on the position of the object.
Convex Mirror
A convex mirror has its reflecting surface curved outward.
Properties:
- Always forms virtual and erect images
- Image is diminished
Uses:
- Rear-view mirrors in vehicles
Convex mirrors provide a wider field of view.
Refraction of Light
Refraction is the bending of light when it passes from one medium to another.
This happens because:
- Speed of light changes in different media
Examples of Refraction
- A stick appears bent in water
- A coin appears raised in water
Refraction explains many optical illusions.
Laws of Refraction
Refraction follows two laws:
- Incident ray, refracted ray, and normal lie in the same plane
- Ratio of sine of angle of incidence to sine of angle of refraction is constant
This constant is called refractive index.
Lenses
A lens is a transparent material bounded by two spherical surfaces.
There are two types:
- Convex lens
- Concave lens
Convex Lens
A convex lens is thicker at the centre.
Properties:
- Converges light rays
- Can form real or virtual images
Uses:
- Magnifying glass
- Cameras
- Microscopes
Concave Lens
A concave lens is thinner at the centre.
Properties:
- Diverges light rays
- Always forms virtual images
Uses:
- Spectacles for short-sightedness
Human Eye
The human eye is a natural optical instrument.
It enables us to see objects by forming images on the retina.
Structure of Human Eye
Important parts:
- Cornea
- Iris
- Pupil
- Eye lens
- Retina
- Optic nerve
Working of Human Eye
- Light enters through cornea
- Eye lens focuses light
- Image is formed on retina
- Optic nerve sends signals to brain
The image formed on retina is real and inverted, but brain interprets it as erect.
Defects of Vision
Common defects:
- Myopia (short-sightedness)
- Hypermetropia (long-sightedness)
These defects are corrected using suitable lenses.
Importance of Optics in Daily Life
Optics helps in:
- Vision correction
- Optical instruments
- Photography
- Communication systems
This chapter is frequently tested in MCQs, diagram-based questions, and concept questions.
FAQs – OPTICS (Exam-Focused)
Does light always travel in a straight line?
Yes, in a uniform medium.
Why do we see shadows?
Because light travels in straight lines.
Do laws of reflection apply to curved mirrors?
Yes.
Which mirror always forms virtual images?
Convex mirror.
Why does a stick appear bent in water?
Due to refraction of light.
Which lens is used to correct myopia?
Concave lens.
Where is image formed in the human eye?
On the retina.
Is image formed on retina erect?
No, it is inverted.
Why are rear-view mirrors convex?
To get a wider field of view.
Does refraction occur without change in medium?
No.
Last-Moment Notes (Cheat Sheet)
OPTICS
- Light travels in straight lines
- Shadow formation proves rectilinear propagation
- Reflection → bouncing back of light
- Angle of incidence = angle of reflection
- Concave mirror → real or virtual image
- Convex mirror → always virtual, diminished
- Refraction → bending of light
- Occurs due to speed change
- Convex lens → converging
- Concave lens → diverging
- Image in eye formed on retina
- Myopia → corrected by concave lens
- Hypermetropia → corrected by convex lens