Practice free Optics (Physics) NEET multiple-choice questions online with instant answers and detailed explanations. No login required.
When referring to the power of a lens, what does 1 Dioptre (D) signify?
The SI unit for power of a lens is dioptre (D): 1D = 1m–1. The power of a lens of focal length of 1 metre is one dioptre.
Consider two thin lenses in contact. If the image formed by the first lens acts as the object for the second lens, what type of object is it for the second lens?
This question tests comprehension of the conceptual simplification used in deriving the combined lens formula. 'It must, however, be borne in mind that formation of image by the first lens is presumed only to facilitate determination of the position of the final image. In fact, the direction of rays emerging from the first lens gets modified in accordance with the angle at which they strike the second lens.'
Which of the following statements about the normal to a spherical reflecting surface is correct?
According to the NCERT text, 'The normal in this case is to be taken as normal to the tangent to surface at the point of incidence. That is, the normal is along the radius, the line joining the centre of curvature of the mirror to the point of incidence.'
When a parallel beam of light, constituted by paraxial rays, is incident on a concave mirror, the reflected rays:
The NCERT text states, 'The reflected rays converge at a point F on the principal axis of a concave mirror [Fig. 9.3(a)].' This point F is defined as the principal focus for a concave mirror.
For a spherical mirror, the relationship between its focal length (f) and radius of curvature (R) for paraxial rays is given by:
The NCERT text explicitly derives and states this relationship: 'We now show that $f = R/2$, where R is the radius of curvature of the mirror.' This derivation is based on the small angle approximation for paraxial rays.
According to the Cartesian Sign Convention for spherical mirrors, if an object is placed to the left of the mirror, the object distance (u) is typically considered:
The Cartesian Sign Convention usually dictates that distances measured against the direction of incident light (which typically comes from the left) are negative. While the provided excerpt specifically mentions 'principal axis (x-axis) of the mirror/lens are taken as positive', the standard convention (implied by Fig 9.2's left-to-right incident light and positive x-axis to the right) is that object distances for real objects placed to the left are negative, as per general physics conventions for optical instruments. The textbook also mentions 'Distances measured in the same direction as the incident light are positive; those measured in the opposite direction are negative.' If incident light is from left to right, and the pole is the origin, then an object to the left would have a negative distance.
An image is considered 'real' if:
The NCERT text defines a real image as: 'The image is real if the rays actually converge to the point; it is virtual if the rays do not actually meet but appear to diverge from the point when produced backwards.'
Which of the following rays is NOT one of the convenient rays typically chosen for drawing ray diagrams for spherical mirrors?
The text lists four convenient rays: (i) parallel to principal axis, (ii) through center of curvature, (iii) through focus, (iv) incident at the pole. A 'ray incident at the edge... far from the pole' is not specifically mentioned as a convenient ray to use for simple ray tracing, especially for paraxial approximations.
If the lower half of a concave mirror's reflecting surface is covered with an opaque material, what effect will it have on the image of an object placed in front of it?
Example 9.1 in the NCERT text states: 'You may think that the image will now show only half of the object, but taking the laws of reflection to be true for all points of the remaining part of the mirror, the image will be that of the whole object. However, as the area of the reflecting surface has been reduced, the intensity of the image will be low (in this case, half).'
For a convex mirror, a parallel beam of paraxial rays incident on it will:
The NCERT text states: 'For a convex mirror, the reflected rays appear to diverge from a point F on its principal axis [Fig. 9.3(b)]. The point F is called the principal focus of the mirror.'
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