Ray Optics and Optical Instruments Class 12 Notes is one of the most important chapters of Class 12 Physics. This chapter deals with the behavior of light when it travels in the form of rays and interacts with mirrors, lenses, and optical instruments. Questions from this chapter are frequently asked in CBSE Board Exams, NEET, JEE (Main) and other competitive exams.
In ray optics, light is treated as a straight-line ray, which helps us understand reflection, refraction, image formation, and magnification. The second part of the chapter focuses on optical instruments like the human eye, microscope, and telescope that help us see objects more clearly.
These notes explain each topic in detail, include important formulas, ray diagram concepts, and are extremely helpful for exam revision.
Ray Optics and Optical Instruments Class 12 Notes PDF | Important & Repeated Questions
1. Nature of Light
Light is a form of energy that produces the sensation of vision. In ray optics:
- Light is assumed to travel in straight lines.
- Each straight-line path is called a ray of light.
- A group of rays is called a beam of light.
Types of Beams:
- Parallel beam – rays are parallel to each other
- Convergent beam – rays meet at a point
- Divergent beam – rays spread out
2. Reflection of Light
Reflection is the phenomenon in which light returns to the same medium after striking a surface.
Laws of Reflection:
- The incident ray, reflected ray, and normal all lie in the same plane.
- The angle of incidence is equal to the angle of reflection.
3. Spherical Mirrors
Mirrors whose reflecting surfaces are spherical are called spherical mirrors.
Types of Spherical Mirrors:
- Concave Mirror – reflecting surface is inward
- Convex Mirror – reflecting surface is outward
Important Terms:
- Pole (P) – center of the mirror
- Centre of Curvature (C) – center of the sphere
- Radius of Curvature (R) – distance between P and C
- Focal Point (F) – point where parallel rays meet
- Focal Length (f) – distance between P and F
Relation:
f = \frac{R}{2}
4. Image Formation by Concave Mirror
Depending on the position of the object:
- Image may be real or virtual
- Image may be erect or inverted
- Image size may be magnified, diminished, or same
Ray diagrams are very important for exams.
5. Mirror Formula and Magnification
Mirror Formula:
\frac{1}{f} = \frac{1}{v} + \frac{1}{u}
Where:
- f = focal length
- v = image distance
- u = object distance
Magnification:
m = \frac{h_i}{h_o} = -\frac{v}{u}
6. Refraction of Light
Refraction is the bending of light when it passes from one medium to another.
Laws of Refraction (Snell’s Laws):
- Incident ray, refracted ray, and normal lie in the same plane.
- \frac{\sin i}{\sin r} = \text{constant}
This constant is called refractive index.
7. Refractive Index
Refractive index (μ) of a medium is defined as:
\mu = \frac{c}{v}
Where:
- c = speed of light in vacuum
- v = speed of light in medium
8. Total Internal Reflection
When light travels from a denser to rarer medium and the angle of incidence is greater than the critical angle, total internal reflection occurs.
Conditions:
- Light must travel from denser to rarer medium.
- Angle of incidence > critical angle.
Applications:
- Optical fiber
- Diamond brilliance
- Mirage
9. Refraction at Spherical Surfaces
When light refracts at a curved surface:
\frac{\mu_2}{v} - \frac{\mu_1}{u} = \frac{\mu_2 - \mu_1}{R}
This formula is important for numerical problems.
10. Thin Lenses
A lens is a transparent medium bounded by two spherical surfaces.
Types of Lenses:
- Convex Lens – converging lens
- Concave Lens – diverging lens
11. Image Formation by Lenses
Convex lens can form:
- Real and inverted images
- Virtual and erect images (when object is between F and O)
Concave lens always forms:
- Virtual, erect, and diminished image
12. Lens Formula and Magnification
Lens Formula:
\frac{1}{f} = \frac{1}{v} + \frac{1}{u}
Magnification:
m = \frac{v}{u}
13. Power of a Lens
Power (P) of a lens is defined as:
P = \frac{1}{f}
Unit: Dioptre (D)
- Convex lens → positive power
- Concave lens → negative power
14. Combination of Lenses
For lenses in contact:
P_{total} = P_1 + P_2 + P_3
15. Optical Instruments
Optical instruments are devices used to improve vision.
16. Human Eye
The human eye acts like a camera.
Important Parts:
- Retina
- Cornea
- Eye lens
- Iris
- Pupil
Defects of Vision:
- Myopia (Short-sightedness) – corrected by concave lens
- Hypermetropia (Long-sightedness) – corrected by convex lens
- Presbyopia – due to aging
17. Simple Microscope
A simple microscope uses a convex lens.
Magnifying Power:
M = 1 + \frac{D}{f}
Where D = least distance of distinct vision (25 cm)
18. Compound Microscope
Uses two lenses:
- Objective lens
- Eyepiece
Magnifying Power:
M = \frac{L}{f_o} \times \frac{D}{f_e}
19. Astronomical Telescope
Used to observe distant objects like stars.
Magnifying Power:
M = \frac{f_o}{f_e}
Conclusion
Ray Optics and Optical Instruments is a concept-based and numerical-rich chapter. Understanding ray diagrams, formulas, and image formation is crucial for scoring high marks. Regular practice of numericals and diagrams will help students master this chapter.
These notes are designed to be clear, exam-oriented, and revision-friendly, making them perfect for Class 12 board exams and competitive exams.
⭐ Important Questions – Ray Optics and Optical Instruments (Class 12)
🔹 1 Mark Questions (Very Short Answer)
- What is the focal length of a plane mirror?
- Define refractive index of a medium.
- What is the unit of power of a lens?
- What type of mirror is used as a rear-view mirror and why?
- What is the least distance of distinct vision for a normal human eye?
- Define critical angle.
- Is magnification of a concave mirror always negative?
- Name the phenomenon responsible for the working of optical fibre.
🔹 2 Mark Questions (Short Answer)
- State the laws of reflection of light.
- Define total internal reflection. Write two conditions for it.
- What is refraction of light? Why does it occur?
- Write the mirror formula and explain the symbols used.
- What is magnifying power of a microscope?
- Distinguish between real image and virtual image.
- Write two uses of convex mirror.
- What is power of a lens? How is it related to focal length?
🔹 3 Mark Questions
- Explain refraction of light through a rectangular glass slab with ray diagram.
- Derive the relation between focal length and radius of curvature of a spherical mirror.
- Explain image formation by a concave mirror when the object is placed beyond C.
- Define refractive index. Write its formula and SI unit.
- Explain myopia and its correction using ray diagram.
- What is total internal reflection? Mention two applications.
- Explain working of simple microscope with diagram.
🔹 5 Mark Questions (Long Answer – Very Important)
- Derive mirror formula for a concave mirror using ray diagram.
- Explain image formation by a convex lens for different object positions with ray diagrams.
- What are the defects of vision? Explain myopia and hypermetropia with ray diagrams and correction.
- Explain the construction and working of an astronomical telescope. Derive expression for its magnifying power.
- Derive lens maker’s formula.
- Explain total internal reflection and its applications in optical fibre.
- Describe human eye. Explain accommodation of eye.
🔁 Repeated / Frequently Asked Questions (Board Exam Oriented)
⚠️ Ye questions pichhle kai saalon se repeat ho rahe hain, isliye inhe skip na karein.
🔸 Repeated 1–2 Mark Questions
- Why convex mirror is used in vehicles?
- Define magnification. Write its formula for mirror and lens.
- What happens to speed of light when it enters glass from air?
- What is critical angle? On what factors does it depend?
- Write sign convention for mirrors.
🔸 Repeated 3 Mark Questions
- Explain total internal reflection with neat ray diagram.
- Explain refraction through prism (deviation of light).
- Draw ray diagram showing image formation by concave mirror when object is at focus.
- Explain correction of presbyopia.
- Write any three uses of total internal reflection.
🔸 Repeated 5 Mark Questions (Most Important)
- Explain defects of vision and their correction with ray diagrams.
- Derive expression for magnifying power of a compound microscope.
- Explain image formation by convex lens with ray diagrams (any three cases).
- Describe working of human eye and its defects.
- Explain astronomical telescope in normal adjustment.
📌 Exam Preparation Tips (Bonus for Students)
✔ Ray diagrams practice daily
✔ All formulas revise regularly
✔ Numerical questions from mirror & lens must solve
✔ Human eye & defects → guaranteed marks
✔ Optical instruments → long answer favorite
We are completed Ray Optics and Optical Instruments Class 12 Notes.