Q.(a) Draw a labelled ray diagram showing the formation of an image by an astronomical refracting telescope in normal adjustment. Hence, obtain the expression for its magnifying power.

Showing the 12 most recent of 36 on this concept.

1. CBSE 2026Set 55/3/11 markMCQ
   Q.A telescope has an objective lens of focal length $144$ cm and an eyepiece of focal length $6.0$ cm. The magnifying power and the length of the telescope tube will be respectively : (A) $24,\ 150$ cm (B) $42,\ 138$ cm (C) $24,\ 138$ cm (D) $42,\ 150$ cm

   ›Reveal solutionSolution

   For a telescope in normal adjustment, the magnifying power is the ratio of objective to eyepiece focal lengths ($M = f_o / f_e$), and the tube length is their sum ($L = f_o + f_e$). Here, $M = 144/6 = 24$ and $L = 144 + 6 = 150$ cm, so the correct option is (A).

   The question gives you a telescope with an objective of focal length $f_o = 144$ cm and an eyepiece of focal length $f_e = 6.0$ cm. You need the magnifying power and the length of the telescope tube.

   The key idea is that a telescope is used to view distant objects. Light from a faraway object arrives as nearly parallel rays. The objective lens forms a real, inverted image at its focal plane. The eyepiece then acts as a magnifier to view that image.

   For relaxed viewing (normal adjustment), the eyepiece is adjusted so that the final image is at infinity. This means the image formed by the objective must lie exactly at the focal point of the eyepiece. So the distance between the two lenses — the tube length — is simply the sum of their focal lengths.

   The magnifying power (or angular magnification) is defined as the ratio of the angle subtended at the eye by the final image to the angle subtended by the object when viewed directly. For a telescope in normal adjustment, this ratio simplifies beautifully to the ratio of the focal lengths.

   For a telescope in normal adjustment:

   $M = \frac{f_o}{f_e} \quad \text{and} \quad L = f_o + f_e$

   Let's apply this directly.

   1. Magnifying power:

   $M = \frac{f_o}{f_e} = \frac{144}{6.0} = 24$

   2. Tube length:

   $L = f_o + f_e = 144 + 6.0 = 150 \text{ cm}$

   So the magnifying power is 24 and the tube length is 150 cm. …

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2. CBSE 2026Set V11 mark
   Q.The bending of light around the corners and entering into the geometric shadow region is called __________. Fill in the blank choosing the appropriate answer from the bracket: (photons, diffraction, polarity, monopoles, greater than unity, less than unity)

   ›Reveal solutionSolution

   diffraction …

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3. CBSE 2026Set ANNUAL1 mark
   Q.Read the following passage carefully and answer the questions given below - Light entering in a dark room through a narrow gap under or around a closed door often appears to bend and spread. This is due to diffraction, the narrow gap acts like a slit, and light wave bends around the edges. This phenomenon is a small but clear demonstration of diffraction.

   (a) What would be the approximate size of sharp edge or opening compared to the wavelength of light for diffraction to be clearly observed?

   ›Reveal solutionSolution

   Diffraction is prominent only when the aperture/obstacle size is comparable to the wavelength of light.

   Diffraction effects become clearly noticeable only when the size of the slit/gap or obstacle is of the same order of magnitude as (comparable to) the wavelength of light used. If the opening is much larger than the wavelength, the bending is negligible and light appears to travel in straight lines (ray optics applies); as the opening size approaches the wavelength, diffraction (bending and spreading) becomes prominent — which is why the narrow …

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4. CBSE 2026Set ANNUAL1 mark
   Q.Read the following passage carefully and answer the questions given below - Light entering in a dark room through a narrow gap under or around a closed door often appears to bend and spread. This is due to diffraction, the narrow gap acts like a slit, and light wave bends around the edges. This phenomenon is a small but clear demonstration of diffraction.

   (b) Why can the diffraction not be explained by ray-optics?

   ›Reveal solutionSolution

   Ray optics assumes light always travels in straight lines and cannot describe the wave interference behind diffraction.

   Ray (geometrical) optics treats light purely as straight-line rays and predicts that an obstacle or slit will simply produce sharp-edged shadows/beams, with no bending at the edges. Diffraction, however, is fundamentally a wave phenomenon — it arises from the superposition (interference) of secondary wavelets originating from different points of the same wavefront as it passes an edge or narrow opening (Huygens–Fresnel principle). Since ray optics ignores the wave nature of …

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5. CBSE 2026Set ANNUAL1 mark
   Q.Define diffraction of light.

   ›Reveal solutionSolution

   Diffraction is the deviation of light from a straight-line path when it passes an obstacle or a narrow opening whose size is comparable to its wavelength.

   Diffraction of light is the phenomenon of bending of light waves around the corners of an obstacle or spreading of light after passing through a narrow slit/aperture, so that light appears in regions that would be a dark geometrical shadow according to simple ray optics. It becomes noticeable when the size of the obstacle/aperture is comparable to the wavelength of light, and is a direct consequence of the wave nature of light (a manifestatio …

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6. CBSE 2026Set ANNUAL1 markMCQ
   Q.The size of the obstacle for the diffraction of light should be

   (a) much larger than the wavelength of light

   (b) much smaller than the wavelength of light

   (c) of the order of wavelength of light

   (d) anything can happen

   ›Reveal solutionSolution

   Diffraction (bending of waves around obstacles) is significant only when the obstacle or slit size is comparable to the wavelength - much bigger or much smaller sizes don't show it clearly.

   Diffraction is the bending/spreading of waves as they pass an obstacle or through an aperture. This spreading is only prominent when the size of the obstacle/aperture (a) is of the SAME ORDER as the wavelength (lambda) of the wave, i.e. a ~ lambda. If the obstacle is much LARGER than the wavelength, the wave essentially travels in straight lines (geometrical shadow, negligible diffraction) - this is why we don't see visible light (wavelength ~500 nm) diffracting around everyday-s …

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7. CBSE 2026Set ANNUAL1 mark
   Q.What do you understand by the term 'diffraction of light'?

   ›Reveal solutionSolution

   Diffraction is the bending of light into the region that geometrical (ray) optics would call 'shadow' when it passes an obstacle or aperture.

   According to simple ray/geometrical optics, light travelling past an obstacle or through a slit should produce a sharp-edged shadow. In reality, being a wave, light bends slightly around the edges of the obstacle/aperture and spreads into what would otherwise be the shadow region, producing a pattern of bright and dark fringes near the edges. This bending and spreading of light waves around obstacles/apertures is called diffraction. It becomes prominent only when the size of the obstacle or aperture is comparable to the wavelength of light, …

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8. CBSE 2026Set ANNUAL1 markMCQ
   Q.Select the correct option with respect to the figures given below:

   (a) Fig.

   (i) depicts diffraction pattern and Fig.

   (ii) depicts interference pattern

   (b) Both depict interference pattern

   (c) Both depict diffraction pattern

   (d) Fig.

   (i) depicts an interference pattern due to a double-slit and Fig.

   (ii) depicts a diffraction pattern due to a single-slit

   ›Reveal solutionSolution

   The key distinguishing feature between interference and diffraction intensity patterns is the relative heights of the fringes: interference from two narrow slits gives many fringes of roughly equal intensity, while diffraction from a single slit gives one strong central maximum with much weaker, rapidly falling secondary maxima.

   Distinguishing interference from diffraction patterns

   In Young's double-slit interference experiment, two coherent narrow sources produce a pattern of bright and dark fringes on the screen. Because the two slits are treated as (nearly) point/line sources of equal amplitude, the resulting bright fringes are all of comparable/roughly equal intensity across the region observed (there is a slowly-varying diffraction "envelope" from each slit's own finite width, but for the idealised interference pattern, the fringes near the centre appear as a series of similar-height peaks, evenly spaced by the fringe width $\beta=\lambda D/d$).

   In single-slit diffraction, light passing through one slit produces a pattern with:

   • one very bright central maximum (roughly twice as wide as the secondary maxima), and
   • a series of much fainter secondary maxima on either side, whose intensities fall off rapidly (roughly as $1/m^2$ or faster) with distance from the centre.

   Applying this to the two figures

   …

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9. CBSE 2026Set ANNUAL1 mark
   Q.Write True or False: The energy redistribution takes place in phenomenon of diffraction.

   ›Reveal solutionSolution

   True — diffraction redistributes light energy from minima to maxima; total energy is conserved.

   Diffraction (like interference) does not create or destroy energy. Where destructive interference produces dark fringes (minima), the energy that would have arrived there is redistributed to the bright fringes (maxima), where constructive interference occurs. The average energy over the w …

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10. CBSE 2025Set 55/4/11 markMCQ
    Q.Assertion (A): In a double slit experiment, if one slit is closed, the diffraction pattern due to the other slit will appear on the screen. Reason (R): For interference, at least two waves are required. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.

    ›Reveal solutionSolution

    The assertion is true — closing one slit leaves a single-slit diffraction pattern — and the reason is also true, because interference requires two coherent waves. But the reason does not explain the assertion; it merely states a necessary condition for interference, not why a single slit produces diffraction. So both are true, but (R) is not the correct explanation of (A). The correct option is (B).

    Let’s unpack this carefully. The question tests your understanding of two distinct phenomena: diffraction and interference, and how they relate in a double-slit experiment.

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    1. What happens when both slits are open?

    In the classic Young’s double-slit experiment, light from a single source passes through two narrow slits. Each slit acts as a coherent secondary source (Huygens’ principle). The waves from the two slits overlap on the screen and interfere — producing alternating bright and dark fringes (interference pattern). But that’s not the whole story.

    Each slit individually also diffracts the light — because the slit width is comparable to the wavelength. So the actual pattern on the screen is a combination: a broad single-slit diffraction envelope modulating the sharp double-slit interference fringes.

    The intensity in a double-slit experiment is:

    $I(\theta) = I_0 \left( \frac{\sin \beta}{\beta} \right)^2 \cos^2 \alpha$

    where $\beta = \frac{\pi a \sin\theta}{\lambda}$ (diffraction factor, $a$ = slit width) and $\alpha = \frac{\pi d \sin\theta}{\lambda}$ (interference factor, $d$ = slit separation).

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    2. Assertion (A): If one slit is closed, the diffraction pattern due to the other slit will appear.

    Yes — this is true. When you block one slit, you are left with a single slit of width $a$. Light passing through that single slit spreads out due to diffraction. The pattern on the screen is a central bright maximum flanked by weaker, narrower secondary maxima — the classic single-slit diffraction pattern.

    Note

    The single-slit diffraction pattern is given by $I(\theta) = I_0 \left( \frac{\sin \beta}{\beta} \right)^2$, where $\beta = \frac{\pi a \sin\theta}{\lambda}$. The first minimum occurs at $\sin\theta = \lambda / a$.

    So the assertion is correct.

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    3. Reason (R): For interference, at least two waves are required.

    This is also true. Interference is the superposition of two or more coherent waves. With one slit, you have only one wavefront emerging — so no interference between two separate sources. You get diffraction, not interference. …

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11. CBSE 2025Set A1 mark
    Q.Fill in the blank with appropriate word: Simple microscope is a converging lens of ______ focal length.

    ›Reveal solutionSolution

    A simple microscope must have a short focal length to give useful magnification.

    A simple microscope is just a single convex (converging) lens used to view a small object placed within its focal length, forming a magnified, virtual, erect image. Its angular magnification (when the image is formed at the near point D = 25 cm) is given by m = 1 + D/f. This shows that the magnification increases as the focal length f decre …

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12. CBSE 2025Set ANNUAL1 mark
    Q.Draw the diagram of intensity distribution of fringes due to diffraction at a single slit.

    ›Reveal solutionSolution

    

    Single-slit diffraction produces one broad, intense central maximum, flanked on both sides by progressively weaker secondary maxima separated by zero-intensity minima.

    A hand-drawn figure cannot be rendered in this text answer, so the pattern is described precisely instead:

    • Plot intensity $I$ (vertical axis) against position/angle $\theta$ (horizontal axis), symmetric about $\theta = 0$.
    • At the centre ($\theta = 0$) there is a broad central maximum of intensity $I_0$ — this is both the tallest and (angularly) the widest peak, twice the angular width of any secondary maximum.
    • On either side, intensity falls to zero at the first minima, located at $\sin\theta = \pm\lambda/a$ (where $a$ is the slit width). …

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