#reflecting telescope

Optical Instruments - Simple and Compound Microscope

Why does the image in a reflecting telescope remain free of chromatic aberration?

**Compound microscope** M = m_o × M_e, objective magnification m_o = v_o/u_o ≈ L/f_o, L tube length, eyepiece M_e = 1+D/f_e (image at D) or D/f_e (infinity). Objective forms real inverted magnified image at focal plane of eyepiece, eyepiece acts as simple microscope magnifying it. A reflecting telescope uses mirrors instead of lenses as the objective. Mirrors reflect all wavelengths of light uniformly, avoiding the wavelength-dependent refraction that causes chromatic aberration in lenses, resulting in a clearer, color-corrected image. Substituting values gives Due to uniform reflection of all

Ref: NCERT > Physics Book > Ray Optics > Optical Instruments - Simple and Compound Microscope

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Telescope, Human Eye and Defects of Vision

Why does a concave mirror used in a reflecting telescope require precise curvature?

**Astronomical telescope** in normal adjustment M = -f_o/f_e, f_o objective focal length (m), f_e eyepiece focal length, length L = f_o+f_e, final image at infinity, angular magnification ratio of angles subtended by image and object. For f_o=100 cm, f_e=5 cm, M=-20, inverted, used for celestial observation. The precise curvature of a concave mirror ensures that all parallel rays from a distant object converge accurately to a single focal point. Any deviation in curvature causes spherical aberration, blurring the image and reducing the telescope’s resolving power. Substituting values gives To

Ref: NCERT > Physics Book > Ray Optics > Telescope, Human Eye and Defects of Vision

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Optical Instruments - Simple and Compound Microscope

Why does a reflecting telescope often use a secondary mirror in its design?

**Simple microscope** magnification M = 1 + D/f when image at D=25 cm near point, M = D/f when image at infinity (relaxed eye), f focal length (cm), D least distance of distinct vision 25 cm. For f=4 cm, image at 25 cm, M=1+25/4=7.25, angular magnification ratio of angle subtended by image to that by object at D. In a reflecting telescope (e.g., Cassegrain design), a secondary mirror redirects the light focused by the primary concave mirror through a hole in the primary mirror to the eyepiece. This allows a compact design with a long focal length, improving magnification and convenience of obs

Ref: NCERT > Physics Book > Ray Optics > Optical Instruments - Simple and Compound Microscope

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Thin Lenses - Lens Formula, Magnification and Power

In a reflecting telescope, why is a concave mirror preferred over a convex lens as the objective?

**Lens formula** 1/f = 1/v - 1/u, f focal length (m), u object distance, v image distance, sign: u negative if object left of lens in Cartesian convention, magnification m = v/u, real image inverted m negative, virtual erect m positive. Power P = 1/f (diopters D), f in meters, +5 D means f=0.2 m=20 cm converging. A concave mirror avoids chromatic aberration (color distortion) that occurs in lenses due to different wavelengths refracting differently. Mirrors reflect all wavelengths uniformly, providing clearer images, and are easier to support mechanically for large apertures, enhancing light-g

Ref: NCERT > Physics Book > Ray Optics > Thin Lenses - Lens Formula, Magnification and Power

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