JEE MAIN - Physics (2020 - 9th January Evening Slot - No. 3)
In a Young's double slit experiment 15 fringes
are observed on a small portion of the screen
when light of wavelength 500 nm is used. Ten
fringes are observed on the same section of
the screen when another light source of
wavelength $$\lambda $$ is used. Then the value of $$\lambda $$ is
(in nm) __________.
Answer
750
Explanation
The length of the screen used portion for 15
fringes, and also for ten fringes
15 $$ \times $$ 500 $$ \times $$ $${D \over \lambda }$$ = 10 $$ \times $$ $${{\lambda D} \over \lambda }$$
$$ \Rightarrow $$ 15 × 50 =$$\lambda $$
$$ \Rightarrow $$ $$\lambda $$ = 750 nm
15 $$ \times $$ 500 $$ \times $$ $${D \over \lambda }$$ = 10 $$ \times $$ $${{\lambda D} \over \lambda }$$
$$ \Rightarrow $$ 15 × 50 =$$\lambda $$
$$ \Rightarrow $$ $$\lambda $$ = 750 nm
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