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Ray Optics: Reflection, Mirrors & Refraction Basics

Detailed revision of Ray Optics covering reflection, plane mirrors, velocity and rotation of images, spherical mirrors, ray diagrams, and basic refraction laws. Includes solved numerical problems and conceptual traps relevant to competitive exams.

JEE
NEET
Physics
English · 11 topics
Checked against the video on 23 Oct 2026

Made by AI from the YouTube video RAY OPTICS in 1 Shot: All Concepts & PYQs Covered || JEE Main & Advanced (opens YouTube in a new tab) by JEE Wallah, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Ray Optics: Reflection, Mirrors & Refraction Basics

English11 topics · 5 MCQs · 8 flashcardsWatch the video (opens YouTube in a new tab)

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In short

Detailed revision of Ray Optics covering reflection, plane mirrors, velocity and rotation of images, spherical mirrors, ray diagrams, and basic refraction laws. Includes solved numerical problems and conceptual traps relevant to competitive exams.

Introduction & Strategy

Ray optics is a lengthy chapter with significant weightage in competitive exams like JEE Main and Advanced.
Optical instruments have been separated from this module to focus systematically on reflection, refraction, prisms, and lenses.
Must knowRay optics typically accounts for around two questions in the JEE Main examination.

Laws of Reflection & Vector Form

When light falls on any surface, three phenomena can occur: reflection (bouncing back), absorption, and refraction (bending across media).
The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane; mathematically, their scalar triple product (box product) is zero.
The angle of incidence is always equal to the angle of reflection (i = r), measured relative to the normal.
Laws of reflection are equally valid for both regular (smooth) and diffused (rough) surfaces.
Must knowIn vector form, the unit vector of the incident ray relates to the reflected ray and normal unit vectors as: I_hat = R_hat + 2(I_hat . n_hat)n_hat.

Plane Mirror: Properties & Clock Problems

In a plane mirror, object distance from the mirror is always equal to image distance (d_object = d_image).
The height of the image equals the height of the object; the image formed is laterally inverted.
Must knowThe plane mirror acts as a perpendicular bisector to the line joining the object and its image.
A real object produces a virtual image in a plane mirror, whereas a virtual object (converging incident rays) produces a real image.
For clock reflection problems, the reflected time is calculated by subtracting the actual time from 11:60 (or 11:59:60 for seconds).

Rotation & Motion of Object and Mirror

When the object is rotated by an angle theta relative to the mirror, the image also rotates by theta but in the opposite sense.
When the mirror is rotated by an angle theta, the image rotates by an angle 2*theta in the same sense.
If an object moves perpendicular to a stationary mirror with velocity v, the image moves with velocity -v (towards or away).
Motion of an object parallel to the mirror results in image motion identical in magnitude and direction.
Must knowIf a mirror moves perpendicular to itself with velocity v towards a stationary object, the image moves with velocity 2v in the same direction with respect to ground.
Motion of the mirror parallel to its own surface causes no change or motion in the image position.

Number of Images Formed by Two Mirrors

To find the number of images formed by two inclined plane mirrors, first calculate n = 360 / theta.
Must knowIf 360 / theta is an even integer, the number of images formed is always (360 / theta) - 1, irrespective of object symmetry.
If 360 / theta is an odd integer and the object is placed symmetrically on the angle bisector, the number of images is (360 / theta) - 1.
If 360 / theta is an odd integer and the object is placed asymmetrically, the number of images is 360 / theta.
Parallel mirrors facing each other (theta = 0) produce an infinite number of images if rays continuously bounce between them.

Field of View & Minimum Mirror Size

The field of view is the physical region bounded by the reflected rays within which an observer can see an image.
A mirror of any size (even point-sized) forms a complete image of an object, though only a small field of view is produced.
Must knowThe minimum vertical height of a plane mirror required for a person of height H to see their full image is H / 2.
To view one's full body, the mirror's lower edge must be positioned at a height of h_eye / 2 above the ground.
To view the complete back wall of a room behind a person situated at distance L from the mirror, similar triangle ratios determine the required mirror height.

Spherical Mirrors: Basics & Focal Length

Spherical mirrors are cut from spherical glass shells; silvered on the outside forms a concave (converging) mirror, while silvered on the inside forms a convex (diverging) mirror.
Must knowThe focal length of a spherical mirror for paraxial rays is f = R / 2, where R is the radius of curvature.
The focal length of a spherical mirror is independent of the surrounding medium (it remains R / 2 in water, oil, or air).
Paraxial rays travel close to the principal axis and make very small angles with the axis, satisfying small-angle approximations.

Ray Diagrams & Graph of v vs u

For a concave mirror, an object at infinity forms a real, inverted, highly diminished image at the focus.
Placing an object at the center of curvature (C) yields a real, inverted image of the same size at C (u = -2f, v = -2f).
Placing an object between the focus (F) and pole (P) in a concave mirror produces a virtual, erect, and magnified image behind the mirror.
In a convex mirror, a real object always forms a virtual, erect, and diminished image between the pole and focus.
Must knowThe v versus u curve for spherical mirrors is a hyperbola intersecting the unit slope line (v = u) at (-2f, -2f) for a concave mirror.

Mirror Formula & Magnifications

The mirror formula relating focal length f, image distance v, and object distance u is: 1/f = 1/v + 1/u.
Cartesian sign convention: all distances are measured from the pole; distances in the direction of incident rays are positive, opposite are negative.
Must knowLateral (transverse) magnification is given by: m_lat = h_image / h_object = -v / u.
Must knowLongitudinal magnification along the principal axis for small objects is given by: m_long = t_image / t_object = -(v² / u²) = -(m_lat)².
A negative magnification indicates an inverted image, while a positive magnification represents an erect image.

Image Velocity for Spherical Mirrors

Component of image velocity parallel to the principal axis: V_image_parallel = m_long × V_object_parallel = -(v² / u²) × V_object_parallel.
Component of image velocity perpendicular to the principal axis: V_image_perp = m_lat × V_object_perp = -(v / u) × V_object_perp.
The total vector velocity of the image is obtained by summing the resolved perpendicular and parallel components.

Refraction, Snell's Law & Lateral Shift

Refraction is the phenomenon of bending of light as it passes across an interface separating two transparent media.
Frequency of light depends purely on the source and never changes across different media during refraction.
Velocity and wavelength change according to refractive index: mu = c / v = lambda_0 / lambda_med.
Must knowSnell's Law states that across an interface: mu_1 × sin(i) = mu_2 × sin(r).
When moving from rarer to denser medium, light bends towards the normal; from denser to rarer, it bends away from the normal.
Must knowPassing through a parallel-sided transparent slab of thickness t produces a lateral shift: Delta = t × sin(i - r) / cos(r).
Emergent ray is parallel to the incident ray when the media on both sides of the parallel slab are identical (i = e).

Key terms

Lateral Inversion: Reversal of left and right orientation in the image formed by a plane mirror.
Field of View: Region enclosed by extreme reflected or refracted rays where the image is visible to an observer.
Paraxial Rays: Rays that travel close to the principal axis making very small angles with it.
Lateral Magnification: Ratio of the height of the image to the height of the object perpendicular to the principal axis.
Longitudinal Magnification: Ratio of the length of the image to the length of the object along the principal axis.
Relative Refractive Index: Refractive index of medium 2 with respect to medium 1 (mu_2 / mu_1 = v_1 / v_2).
Lateral Shift: Perpendicular distance between the original incident path and the emergent ray after passing through a slab.

Formulas

I_hat = R_hat + 2(I_hat . n_hat)n_hat: Vector relation between incident ray, reflected ray, and normal
Delta = 180 - 2i: Angle of deviation for reflection from a single plane mirror
1/f = 1/v + 1/u: Mirror formula relating focal length, object distance, and image distance
m_lat = -v / u = h_image / h_object: Lateral magnification for spherical mirrors
m_long = -(v² / u²): Longitudinal magnification for small objects along the principal axis
mu = c / v = lambda_0 / lambda_med: Definition of absolute refractive index
mu_1 × sin(i) = mu_2 × sin(r): Snell's Law of refraction
Delta = t × sin(i - r) / cos(r): Lateral shift produced by a parallel slab of thickness t

Quick revision

Read these the night before the exam.

The focal length of a spherical mirror is f = R / 2 and is completely independent of the medium.
A plane mirror requires a minimum vertical height of H / 2 to see the full body of a person of height H.
Rotating a mirror by an angle theta rotates the reflected ray by 2*theta in the same direction.
If 360 / theta is even, the number of images formed by two inclined plane mirrors is always (360 / theta) - 1.
During refraction, the frequency of light remains constant while its velocity and wavelength change.
Longitudinal magnification for differential objects is related to lateral magnification by m_long = -(m_lat)².
Parallel displacement (lateral shift) across a slab leaves the emergent ray parallel to the incident ray if bounding media are identical.
The v versus u graph for a concave mirror passes through the point (-2f, -2f).

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