Light MCQ Class 7

Explanation: Different colours in white Light have distinct wavelengths, but when travelling through a uniform medium like air, their speeds remain nearly identical because air is non-dispersive. The optical properties of a medium, particularly its refractive index, determine Light speed, and in air, this index barely varies across visible wavelengths. In dispersive media such as glass, different colours separate due to varying speeds, but in air, the effect is negligible. Conceptually, this is like runners on a smooth track; their shoe colour does not affect their pace. The underlying idea is that homogeneous media do not differentiate between wavelengths, so all light colours propagate at the same velocity. This explains why sunlight does not split into separate colours in air alone.

Explanation: Total internal reflection occurs when light moves from a denser medium to a rarer one at an angle exceeding the critical angle, reflecting entirely inside the denser medium. This principle explains phenomena like mirages in deserts and the functioning of optical fibres. In mirages, temperature gradients create layers of air with different densities, bending light and producing apparent water-like reflections. In optical fibres, light remains trapped inside the core due to repeated total internal reflections, allowing efficient long-distance transmission with minimal energy loss. It is similar to a ball bouncing inside a tube; thrown at the correct angle, it never escapes. Understanding this effect highlights how light can be confined in specific pathways for technological and natural optical applications.

Explanation: When light transitions from air (a rarer medium) to glass (a denser medium), its speed decreases while its frequency remains unchanged. The wavelength, however, becomes shorter due to the slower speed in the denser medium. This change occurs because the refractive index of glass is higher than that of air. The principle ensures energy conservation and wave continuity at the boundary. A simple analogy is a car moving from a smooth road onto sand; its speed reduces, but the engine’s rhythm (analogous to frequency) remains constant. Understanding this concept is essential in Optics for lens behaviour, refraction, and designing instruments like microscopes and telescopes.

Explanation: The refractive index depends on the optical nature of the medium and the wavelength (colour) of the incident light. Light of shorter wavelengths, like violet, bends more strongly compared to longer wavelengths, like red. While the angle of incidence influences the direction of refracted light, it does not alter the medium’s inherent refractive index. This principle explains why prisms disperse white light into colours and why different glasses or liquids bend light differently. Conceptually, refractive index can be thought of as a measure of “optical density,” affecting how much light slows down and changes direction when entering a medium.

Explanation: Hypermetropia, or farsightedness, is a condition where the eye focuses images behind the retina. This occurs because the eyeball is shorter than normal or the lens is less convex, making nearby objects appear blurry. The far point moves further away, while the near point increases, meaning close objects cannot be seen clearly without correction. The concept relies on understanding image formation in convex lenses and the eye’s optical system. A helpful analogy is a camera with its lens focused too far; nearby subjects appear out of focus. Knowledge of this principle is crucial for designing corrective lenses for near vision.

Explanation: Objects appear shifted due to the refraction of light at the water–air interface. Light bends towards the normal when entering water and away when exiting, making submerged objects seem closer to the surface or higher than their actual position. The perceived position depends on the observer’s viewpoint—above or below water. Understanding Snell’s Law, which relates the angle of incidence and refraction via the refractive indices, helps predict these visual effects. This phenomenon is widely observed in swimming pools or ponds. Conceptually, it is like peering through a transparent plastic sheet; objects seem displaced due to bending of light rays.

Explanation: Light and other electromagnetic waves are transverse waves that carry energy and can be reflected, refracted, or absorbed. Radio waves are generated by oscillating electric currents and are widely used in Communication and speed detection. Light travels slower in denser media like water than in air, and shorter wavelengths scatter more effectively, explaining phenomena like the blue sky. Understanding these general properties is fundamental in Optics, wave mechanics, and electromagnetic applications. Analogy: electromagnetic waves behave like ripples on a pond, spreading energy while interacting with different obstacles.

Explanation: The question involves additive and subtractive colour mixing principles. When coloured filters or paints overlap, certain wavelengths combine or cancel out. Magenta and yellow produce red in the overlapping region, while cyan and magenta produce blue. Understanding primary and secondary colours in light and pigment is necessary to predict the resulting colours. The phenomenon can be visualised with colour wheels or by using simple overlapping coloured transparencies. This principle is applied in printing, painting, and digital displays where combinations of colours produce new hues.

Explanation: The blue colour of the sea arises due to the scattering of sunlight by water molecules, which preferentially scatter shorter wavelengths (blue) more than longer ones (red). Additionally, water absorbs longer wavelengths, enhancing the perception of blue. Reflection of the sky may contribute, but the primary reason is Molecular scattering. This is similar to why the sky appears blue; shorter wavelengths scatter more in the Atmosphere. Understanding this effect requires knowledge of light–Matter interaction and wavelength-dependent scattering, which is central to Optics and environmental Physics.

Explanation: This question tests understanding of sound wave properties. sound waves in gases are longitudinal, with compressions and rarefactions along the direction of propagation. Frequencies below 20 Hz are infrasound, not ultrasonic, which is above 20 kHz. Higher amplitude correlates with louder perception, and higher frequency affects pitch. Understanding these fundamentals requires knowledge of wave mechanics, sound propagation, and auditory perception. Analogously, a slinky stretched and compressed illustrates longitudinal motion and how amplitude and frequency relate to loudness and pitch.

Explanation: The sky appears blue because shorter wavelengths of light, like blue, scatter more strongly than longer wavelengths when sunlight passes through the Atmosphere. Without an Atmosphere, light would not scatter, and the sky would appear black. Red light scatters less than blue, which is why sunrise and sunset appear reddish. This principle relies on Rayleigh scattering, which depends on the inverse fourth power of wavelength. Understanding scattering helps explain natural colour phenomena in the sky and the differences in sky colour under various atmospheric conditions.

Explanation: Sunlight consists of ultraviolet (UV), visible, and infrared radiation. Ultraviolet rays carry higher energy and can penetrate the skin, affecting cellular structures and triggering melanin production, leading to tanning. Excessive UV exposure can damage skin cells, causing sunburn. Visible light and infrared primarily contribute to illumination and Heat but do not cause tanning. This concept is crucial for understanding human Health, sunscreen protection, and the effects of Solar radiation. An analogy is a strong beam of energetic particles affecting a sensitive surface while weaker light only illuminates it.

Explanation: Diffraction patterns depend on the wavelength of light; shorter wavelengths produce narrower patterns while longer wavelengths spread out more. Replacing red light with blue light decreases the wavelength, causing the bands in the Diffraction pattern to move closer together. The underlying principle relies on the relationship between wavelength, slit spacing, and angular spread of Diffraction maxima. Understanding this effect is essential in Optics experiments, like single-slit or double-slit setups, where precise measurements depend on wavelength-dependent Diffraction. Conceptually, it’s like narrowing a wave in a ripple tank, which reduces the spread of interference fringes.

Explanation: The question tests knowledge of Optics and human vision. Light from the sun reaches Earth in about 8 minutes and 20 seconds. Convex lenses magnify objects placed between their focus and optical centre, while concave lenses reduce image size. Persistence of vision explains motion perception in cinematography. Recognising which statement contradicts established Physics or optical principles requires understanding lens behaviour and human visual processing. An analogy is checking a SET of rules against observed behaviour; any mismatch indicates the incorrect statement.

Explanation: A concave mirror forms images whose size depends on the object’s distance relative to its focal point and centre of curvature. When the object is placed at the centre of curvature, the image forms at the same distance on the opposite side of the mirror and has equal size. Placing the object elsewhere alters magnification. This principle is based on the mirror equation and the relationship between object distance, image distance, and focal length. Analogy: It’s like placing a ball at the midpoint of a curved reflector so that its reflection maintains the same size.

Explanation: The magnification of a concave mirror increases as the object moves closer to the focal point. When the object lies between the focal point and the mirror, the image appears enlarged and virtual. This behaviour follows the mirror equation and the sign conventions for concave mirrors. The closer the object to the focal point, the larger the virtual image. A practical analogy is looking into a concave shaving mirror; objects appear bigger when held close to the mirror. Understanding this principle is crucial for magnifying Optics and optical devices like telescopes.

Explanation: Lens power (in dioptres) is inversely proportional to its focal length in metres. Positive power indicates a convex lens, negative power indicates a concave lens. A lens with a given dioptre value will have a predictable focal length based on the relationship P = 100/f (cm) or P = 1/f (m). Understanding this principle allows calculation of focal lengths for vision correction, cameras, and optical instruments. Analogously, stronger lenses have shorter focal lengths and bend light more sharply, while weaker lenses have longer focal lengths.

Explanation: The question involves mirror magnification and the mirror equation. The magnification formula (m = image height/object height = -v/u) relates the sizes of image and object to the object and image distances. Using the image and object heights along with the given image distance, the object distance can be calculated, and then the mirror equation (1/f = 1/u + 1/v) can determine the focal length. This approach applies to all concave or convex mirrors and helps in practical Optics experiments. Analogy: It’s like adjusting the distance of a projector from a screen to get a desired image size.

Explanation: This phenomenon occurs due to a mirror with different curvatures in vertical sections: convex mirrors reduce image size, concave mirrors enlarge it, and plane mirrors maintain original size. Placing concave at the top enlarges the head, plane in the middle preserves the midsection, and convex at the bottom reduces leg size. This demonstrates the effects of mirror curvature on image magnification and reflection. Conceptually, it shows how light rays bend differently based on the mirror’s shape, producing distorted yet predictable images.

Explanation: The CD surface has microscopic grooves that act as a Diffraction grating. When sunlight hits these grooves, it undergoes reflection and Diffraction, separating white light into its component colours. The interference of diffracted light creates the rainbow-like pattern. This is a practical demonstration of light wave behaviour, including diffraction, interference, and the wavelength-dependent separation of colours. Analogy: It’s similar to sunlight passing through a prism, where the arrangement of grooves on the CD splits light into visible colours.

Explanation: Sound is a mechanical wave that requires a medium to travel. Its speed is fastest in Solids, slower in liquids, and slowest in gases because particle density and elasticity affect transmission. Sound cannot travel in a vacuum, which is why talking on the Moon is impossible. The amplitude of a wave determines loudness, and frequency determines pitch. Understanding these principles is essential in acoustics, engineering, and Physics experiments. Analogy: Vibrations travel quickly through a tightly packed crowd (Solid) but more slowly through a sparse group (gas).

Explanation: Rainbows form due to the combined effects of refraction, dispersion, and reflection. Sunlight entering a water droplet bends (refraction), separates into its component colours (dispersion), and reflects internally before emerging. The combination of these effects produces the visible Spectrum in an arc. The colour sequence and angle depend on wavelength, with shorter wavelengths bending more. This principle illustrates the interaction of light with small spherical particles and explains natural optical phenomena. Analogy: Water droplets act as tiny prisms, splitting sunlight into colours like a series of miniature glass prisms.

Explanation: All three types of waves can reflect and transfer energy through their respective media. Electromagnetic waves can propagate through a vacuum, while sound and water waves require a material medium. Waves exert pressure and influence surroundings when they carry energy, but their mechanisms differ. Understanding wave behaviour across types helps compare phenomena like reflection, energy Transport, and interference. Analogy: A ripple in a pond, a sound pulse, and a light beam all carry energy and interact with boundaries, though only some require a medium.

Explanation: In a compound microscope, light first passes through the condenser to focus on the specimen, then enters the objective lens which produces a magnified real image. The eyepiece further magnifies this image for observation. Proper light path ensures clarity, brightness, and resolution. Misalignment of these components can reduce image quality. This process is based on lens Optics, refraction, and magnification principles. Analogy: It’s like projecting an image onto a screen using two sets of magnifying lenses to enlarge fine details.

Explanation: This situation indicates myopia (nearsightedness), where distant objects appear blurry because the eye focuses images in front of the retina. The eyeball may be elongated or the lens too strong, affecting image formation for far objects. Near objects appear clear because they fall within the eye’s focused range. Understanding the optical behaviour of the eye helps in prescribing corrective lenses. Analogy: It is like a camera lens focused too close; distant subjects appear out of focus while nearby ones remain sharp.

Explanation: The apparent shallowness of a swimming pool is due to the refraction of light as it passes from water to air. Light rays bend away from the normal at the water’s surface, causing objects to appear closer to the observer than they actually are. This visual distortion is predictable using Snell’s Law and the refractive indices of water and air. Analogy: It is like viewing an object through a glass sheet at an angle; the image seems displaced due to bending of light. Understanding this effect is essential in Optics and underwater perception studies.