Young's double slit experiment: Class 12 practice
Fringe width β = λD/d. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Scientific notation
The idea behind young's double slit experiment
Fringe width β = λD/d. The nth bright fringe is at yₙ = nλD/d from the centre; dark fringes are halfway between. Immersing the setup in a liquid of index n shrinks β by n.
Step-by-step method
- Convert λ (nm), d (mm) and D (m) to metres.
- β = λD/d.
- yₙ = nβ for bright fringes.
Worked example 1 (Basic)
Find the fringe width for λ = 650 nm, D = 1.9 m, d = 1 mm.
- β = λD/d
- = 1.235 mm
- Answer: 1.235 mm
Worked example 2 (Standard)
For λ = 500.00000000000006 nm, D = 1.1 m and d = 0.30000000000000004 mm, how far from the centre is the 2th bright fringe?
- yₙ = nβ = 2 × 1.833 mm
- = 3.667 mm
- Answer: 3.667 mm
Worked example 3 (Challenge)
A double-slit setup with λ = 650 nm, D = 1.6 m and d = 0.9000000000000001 mm is immersed in water (n = 1.33). Find the new fringe width.
- λ' = λ/1.33 → β' = β/1.33
- = 0.869 mm
- Answer: 0.869 mm
Common mistakes and misconceptions
- Not converting units
- Mixing up D and d
Practice questions
- (Basic) Find the fringe width for λ = 550 nm, D = 1.6 m, d = 1 mm.
- (Basic) Find the fringe width for λ = 600.0000000000001 nm, D = 0.9 m, d = 0.30000000000000004 mm.
- (Standard) For λ = 500.00000000000006 nm, D = 0.7 m and d = 0.4 mm, how far from the centre is the 5th bright fringe?
- (Standard) For λ = 500.00000000000006 nm, D = 1.5 m and d = 0.6000000000000001 mm, how far from the centre is the 3th bright fringe?
- (Challenge) A double-slit setup with λ = 500.00000000000006 nm, D = 1.9 m and d = 0.9000000000000001 mm is immersed in water (n = 1.33). Find the new fringe width.
- (Challenge) A double-slit setup with λ = 600.0000000000001 nm, D = 1.8 m and d = 0.8 mm is immersed in water (n = 1.33). Find the new fringe width.
Answer key with full working
- Q1: β = λD/d → = 0.880 mm → Answer: 0.880 mm
- Q2: β = λD/d → = 1.800 mm → Answer: 1.800 mm
- Q3: yₙ = nβ = 5 × 0.875 mm → = 4.375 mm → Answer: 4.375 mm
- Q4: yₙ = nβ = 3 × 1.250 mm → = 3.750 mm → Answer: 3.750 mm
- Q5: λ' = λ/1.33 → β' = β/1.33 → = 0.794 mm → Answer: 0.794 mm
- Q6: λ' = λ/1.33 → β' = β/1.33 → = 1.015 mm → Answer: 1.015 mm
Syllabus note
This skill is listed under "Wave Optics" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is young's double slit experiment in?
"Wave Optics", Class 12 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.