#real image

Telescope, Human Eye and Defects of Vision

What is the primary reason a concave lens cannot form a real image?

**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. A concave lens diverges light rays, preventing them from converging to a point on the opposite side. The rays appear to diverge from a virtual focal point on the same side as the object, resulting in a virtual image that cannot be projected, regardless of object position. Substituting valu

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

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Combination of Lenses and Lens Systems

Why does a concave mirror form a real image when the object is placed beyond the focal point?

**Two lenses in contact** behave as single lens with power sum, magnification product m = m₁×m₂. For f₁=20 cm (P₁=5 D), f₂=-20 cm (P₂=-5 D) in contact, P=0, F infinite, afocal system, beam emerges parallel, principle of corrective lenses for myopia/hypermetropia. When the object is beyond the focal point of a concave mirror, the reflected rays converge to a point on the same side as the object. This convergence of actual rays results in a real image that can be projected onto a screen, typically inverted relative to the object. Substituting values gives Due to rays converging to a point after

Ref: NCERT > Physics Book > Ray Optics > Combination of Lenses and Lens Systems

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Refraction Through Prism and Minimum Deviation

In a convex lens, what happens to the image if the object is placed between the focal point and twice the focal length?

**Refraction through prism** deviation δ = i+e-A, minimum when i=e, symmetrical path, r₁=r₂=A/2, n = sin[(A+δ_m)/2]/sin(A/2). In water n_rel = n_prism/n_water =1.5/1.33=1.128, so δ_m reduces because relative index lower, new δ_m from formula with n_rel. For a convex lens, when the object is between the focal point (F) and twice the focal length (2F), the image is real, inverted, and magnified. It forms beyond 2F on the opposite side, as the rays converge after refraction. Substituting values gives Real, inverted, and magnified, which matches expected image position and magnification from mirro

Ref: NCERT > Physics Book > Ray Optics > Refraction Through Prism and Minimum Deviation

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Total Internal Reflection and Critical Angle

In a concave mirror, when the object is placed at the center of curvature, where is the image formed?

**Critical angle** C satisfies sinC = n₂/n₁, n₁>n₂, n₂=1 for air, n₁=1.52 for glass gives sinC=1/1.52=0.6579, C≈41.1°, for water n=1.33 C=48.75°. Beyond C, total internal reflection occurs, all light reflected, no refracted ray, used in optical fibers and prisms. For a concave mirror, when the object is at the center of curvature (C), the reflected rays converge back to the same point after reflection. This results in a real, inverted image formed at the center of curvature, with the same size as the object. Substituting values gives At the center of curvature, which matches expected image pos

Ref: NCERT > Physics Book > Ray Optics > Total Internal Reflection and Critical Angle

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Refraction at Plane Surfaces and Snell's Law

In a convex lens, why does the image transition from virtual to real as the object moves from inside to outside the foca

**Refractive index** n = c/v, water 1.33 means light 1.33 times slower than vacuum. Passing from water to air at 49°, n₁ sinθ₁ =1.33×sin49°≈1.33×0.755=1.004>1, so sinθ₂>1 impossible, total internal reflection occurs, no refraction. Inside the focal point, a convex lens diverges rays, forming a virtual image on the same side. Beyond the focal point, the lens converges rays to a point on the opposite side, forming a real image. This transition occurs as the object crosses the focal point, changing the ray behavior. Substituting values gives Due to change from divergence to convergence, which mat

Ref: NCERT > Physics Book > Ray Optics > Refraction at Plane Surfaces and Snell's Law

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Thin Lenses - Lens Formula, Magnification and Power

A convex lens of focal length \( 20 \, \text{cm} \) forms a real image at \( 60 \, \text{cm} \) from the lens. What is t

**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. Focal length: f = 20 cm . Image distance: v = 60 cm (real image). Lens formula: (1/v) - (1/u) = (1/f) . (1/60) - (1/u) = (1/20) ⇒ (1/u) = (1/60) - (1/20) = (1 - 3/60) = (-2/60) = (-1/30) . u = -30 cm .

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

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Reflection by Spherical Mirrors and Mirror Formula

Why does the image formed by a convex lens become real and inverted when the object is moved beyond the focal point?

**Concave mirror image formation** depends on object position: beyond C real inverted diminished between F and C, at C real inverted same size at C, between C and F real inverted magnified beyond C, at F image at infinity, within F virtual erect magnified behind mirror. Beyond the focal point, a convex lens converges light rays to a point on the opposite side of the lens. This convergence forms a real image, and because the rays cross over, the image is inverted relative to the object’s orientation. Substituting values gives Due to convergence of rays on the opposite side, which matches expect

Ref: NCERT > Physics Book > Ray Optics > Reflection by Spherical Mirrors and Mirror Formula

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Thin Lenses - Lens Formula, Magnification and Power

Why does a convex lens form a real image only when the object is outside the focal point?

**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. Outside the focal point, a convex lens converges rays to a point on the opposite side, forming a real image. Inside the focal point, rays diverge after refraction, appearing to come from a point on the same side, resulting in a virtual image instead. Substituting values gives Due

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

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