Misconception 1: "Concave mirror always forms a real image"

Truth: When the object is between the pole P and the focus F, the concave mirror forms a virtual, erect, magnified image (like a shaving mirror). Real images are formed only when the object is beyond F.

Misconception 2: "A convex lens always converges light to a real focus"

Truth: When a convex lens is placed in a denser medium (n_lens < n_medium), it becomes a diverging lens. For example, a glass lens (n=1.5) in carbon disulfide (n=1.63) becomes diverging.

Misconception 3: "TIR means the light is absorbed at the interface"

Truth: TIR is total reflection — no light is transmitted or absorbed. The intensity of the reflected light equals the incident light intensity. This is why optical fibres can transmit signals over great distances without loss.

Misconception 4: "The focal length of a lens changes with the object distance"

Truth: For a thin lens, the focal length is a fixed property of the lens (depends on n, $R_{1}$, $R_{2}$ via lensmaker's equation). It does not change with object position. What changes is the image distance v for different object distances u.

Misconception 5: "Magnification m = 2 always means a real, magnified image"

Truth: m = +2 means erect and magnified. For mirrors, erect images are virtual (not real). For lenses, a virtual, erect, magnified image (m = +2) is formed when the object is within the focal length.

Misconception 6: "The power P = $P_{1}$ + $P_{2}$ rule always applies"

Truth: This rule is valid ONLY for thin lenses in contact. For separated lenses at distance d: P = $P_{1}$ + $P_{2}$ − d·$P_{1}$·$P_{2}$. Using the simpler formula for separated lenses gives wrong results.

Misconception 7: "A prism always shows minimum deviation"

Truth: Minimum deviation is a specific condition (i = e) that occurs at one particular angle of incidence. For other angles of incidence, the deviation is greater than the minimum. The minimum deviation condition is used for measuring refractive index accurately.

Misconception 8: "Higher refractive index always means more deviation by a prism"

Truth: Higher n means a larger deviation AND a larger critical angle change. But deviation also depends on the prism angle A and the incident angle. A thin prism (small A) of high-n glass may deviate less than a thick prism (large A) of lower-n glass.