Q.The focal length of a lens is minimum for which colour?

Showing the 12 most recent of 36 on this concept.

1. CBSE 2026Set A1 markMCQ
   Q.The ratio of the refractive index of red light to blue light in air is (A) less than unity (B) greater than unity (C) equal to unity (D) none of these

   ›Reveal solutionSolution

   Because refractive index rises from red to blue (dispersion), n_red < n_blue, so their ratio is less than 1.

   Dispersion arises because the refractive index of a medium depends on wavelength: it is larger for shorter wavelengths (blue) and smaller for longer wavelengths (red). Thus $n_{red}<n_{blue}$, and their ratio is

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2. CBSE 2026Set ANNUAL1 markMCQ
   Q.The focal length of a lens is minimum for which colour?

   (a) Red

   (b) Yellow

   (c) Violet

   (d) Blue

   ›Reveal solutionSolution

   Since refractive index decreases from violet to red (dispersion), and $f\propto 1/(\mu-1)$, the colour with the highest $\mu$ — violet — bends most and has the smallest focal length.

   For a thin lens, the lens-maker's formula gives $\dfrac{1}{f}=(\mu-1)\left(\dfrac{1}{R_1}-\dfrac{1}{R_2}\right)$, so $f\propto\dfrac{1}{\mu-1}$: a larger refractive index means a smaller focal length. Because the refractive index of ordinary glass is greater for shorter wavelengths, the order from lowest to highest $\mu$ across the visible spectrum is red < orange < yellow < green < blue < …

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3. CBSE 2026Set ANNUAL1 markMCQ
   Q.The speed of light in an isotropic medium depends on :

   (a) the nature of propagation

   (b) its intensity

   (c) the motion of the source with respect to medium

   (d) its wavelength

   ›Reveal solutionSolution

   A medium's refractive index (and hence the speed of light within it) varies with wavelength -- the phenomenon of dispersion.

   Working

   The speed of light in a medium is $v=c/n$. For any real (non-vacuum) isotropic medium, the refractive index $n$ is not fixed but depends on the wavelength of the light (e.g., violet light travels slightly slower than red light in glass) -- this dependence is the cause of dispersion (splitting of white light by a prism).

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4. CBSE 2025Set ANNUAL1 markMCQ
   Q.In glass, the velocity of the light is minimum for :

   (a) red

   (b) violet

   (c) yellow

   (d) green

   ›Reveal solutionSolution

   Violet light bends the most and travels slowest in glass because glass has its highest refractive index for violet light.

   Speed of light in a medium is $v = c/n$. Due to dispersion, the refractive index of glass is different for different colours and increases from red to violet (violet has the shortest wavelength and is refracted most). Since $n_{violet}$ is the largest …

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5. CBSE 2025Set ANNUAL1 markMCQ
   Q.If red light and violet rays are of focal lengths f_r and f_v, then which one of the following is true ?

   (a) lambda_r <= lambda_v

   (b) lambda_r = lambda_v

   (c) mu_r > mu_v

   (d) mu_r < mu_v

   ›Reveal solutionSolution

   Violet light has a shorter wavelength and is refracted more, so its refractive index is higher than red light's.

   By Cauchy's dispersion relation, the refractive index of a medium decreases as wavelength increases: $\mu = A + \dfrac{B}{\lambda^2}$ (approximately). Since red light has a longer wavelength than violet light ($\lambda_r > \lambda_v$), red light is refracted less and has a smaller refractive index than violet light.

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6. CBSE 2025Set ANNUAL1 markMCQ
   Q.If the refractive index of a material of equilateral prism is sqrt(3), then the angle of minimum deviation is

   (i) 30 degrees

   (ii) 45 degrees

   (iii) 60 degrees

   (iv) 75 degrees

   ›Reveal solutionSolution

   Dm = 60 degrees for an equilateral prism with mu = sqrt(3).

   The prism formula is $\mu = \frac{\sin\!\left(\frac{A + D_m}{2}\right)}{\sin\!\left(\frac{A}{2}\right)}$. For an equilateral prism $A = 60^\circ$, so $\sin(A/2) = \sin 30^\circ = 0.5$. Then …

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7. CBSE 2024Set FS1 markMCQ
   Q.The maximum focal length of convex lens is for:

   (i) blue light

   (ii) green light

   (iii) red light

   (iv) yellow light

   ›Reveal solutionSolution

   $\dfrac{1}{f}\propto(n-1)$; red light has the least refractive index in glass, so it gives the greatest focal length — option (iii).

   Concept. For a lens, $\dfrac{1}{f}=(n-1)\left(\dfrac{1}{R_1}-\dfrac{1}{R_2}\right)$, so $f$ is largest when the refractive index $n$ is smallest.

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8. CBSE 2024Set A1 markMCQ
   Q.The wavelength of which colour is minimum? (A) Violet (B) Yellow (C) Blue (D) Red

   ›Reveal solutionSolution

   Violet has the minimum wavelength in the visible spectrum.

   In the visible spectrum (VIBGYOR), wavelength increases steadily from violet to red:

   $\text{Violet} (\sim 400\,\text{nm}) < \text{Blue} < \text{Yellow} < \text{Red} (\sim 700\,\text{nm}).$

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9. CBSE 2024Set A1 markMCQ
   Q.Which causes the formation of rainbow? (A) Diffraction (B) Scattering (C) Refraction (D) Dispersion

   ›Reveal solutionSolution

   A rainbow is produced by dispersion of sunlight inside raindrops → option (D).

   When white sunlight enters a spherical raindrop, it refracts, undergoes total internal reflection at the back of the drop, and refracts again as it leaves. Because the refractive index of water is slightly different for each colour (violet bends most, red least), the white light is separated into its constituent colours. This splitting of white light into it …

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10. CBSE 2024Set ANNUAL1 markMCQ
    Q.With increase in wavelength, the value of refractive index

    (a) increases

    (b) decreases

    (c) remains unchanged

    (d) none of these

    ›Reveal solutionSolution

    In ordinary transparent media, the refractive index falls off with increasing wavelength - this normal dispersion is why a prism spreads white light into a spectrum with violet deviating most and red least.

    For most transparent materials in the visible range, the refractive index depends on wavelength approximately as (Cauchy's empirical formula):

    $n(\lambda) = A + \dfrac{B}{\lambda^2} + \ldots$

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11. CBSE 2024Set ANNUAL1 markMCQ
    Q.Read the following statements carefully : Assertion (A) : When a narrow beam of white light passes through a glass prism, it undergoes dispersion. Reason (R) : The refractive index of medium for different wavelengths (colours) is different. Select the correct answer from the options given below :

    (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).

    (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

    (c) Assertion (A) is true, but Reason (R) is false.

    (d) Both Assertion (A) and Reason (R) are false.

    ›Reveal solutionSolution

    Dispersion happens precisely BECAUSE the prism's refractive index is different for different colours — Reason directly explains Assertion.

    Assertion (A): When white light passes through a glass prism, it splits into its constituent colours (dispersion) — this is a well-established, true fact (seen as a rainbow-like spectrum emerging from the prism).

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12. CBSE 2024Set ANNUAL1 markMCQ
    Q.A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the glass slab, the first colour to emerge is :

    (a) blue

    (b) green

    (c) violet

    (d) red

    ›Reveal solutionSolution

    Red light has the lowest refractive index in glass among visible colours, so it travels fastest through the slab and exits first.

    The refractive index of glass is different for each colour (wavelength) of light — this is dispersion. Violet light has the shortest wavelength and the highest refractive index in glass, while red light has the longest wavelength and the lowest refractive index (by Cauchy's relation, $n$ decreases as $\lambda$ increases).

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