Optics and Refraction — MCQs

$Optics and Refraction — MCQs$

Optics and Refraction Indian Medical PG Practice Questions and MCQs

Question 311: Treatment of presbyopia is by use of which type of lens?

A. Convex (Correct Answer)
B. Concave
C. Biconcave
D. Concavoconvex

Explanation: **Convex** - **Presbyopia** is an age-related condition where the **lens stiffens**, impairing its ability to accommodate and focus on near objects. - **Convex lenses** add converging power to the eye, helping to bring near objects into focus on the retina. *Concave* - **Concave lenses** diverge light rays and are used to correct **myopia (nearsightedness)**, where the eye focuses images in front of the retina. - They spread light out before it enters the eye, pushing the focal point back onto the retina. *Biconcave* - **Biconcave lenses** are a type of concave lens with two concave surfaces, used for correcting severe **myopia (nearsightedness)**. - These lenses further diverge light rays and are not suitable for presbyopia, which requires converging power. *Concavoconvex* - A **concavoconvex lens** has one concave and one convex surface; its overall power depends on the relative curvatures of the two surfaces. - While some forms might be used in specialized optical systems, they are not the primary or standard correction for presbyopia, which typically requires a simple converging (convex) power.

Question 312: Keratometry is useful in measuring:

A. Corneal curvature (Correct Answer)
B. Corneal thickness
C. Corneal diameter
D. Depth of anterior chamber

Explanation: ***Corneal curvature*** - **Keratometry** directly measures the curvature of the central anterior corneal surface, which is crucial for assessing **astigmatism** and fitting **contact lenses**. - The device projects an illuminated object onto the cornea and measures the size of the reflected image to calculate the radius of curvature. *Corneal thickness* - **Corneal thickness** is measured by **pachymetry**, not keratometry. - This measurement is important for diagnosing conditions like **corneal edema** and for glaucoma management (e.g., central corneal thickness influencing intraocular pressure readings). *Corneal diameter* - **Corneal diameter** is typically measured with a ruler or **calipers**, or imaging techniques like **optical coherence tomography (OCT)**, not a keratometer. - This measurement, often referred to as **horizontal visible iris diameter (HVID)**, is mainly relevant for contact lens fitting and refractive surgery planning. *Depth of anterior chamber* - The **depth of the anterior chamber** is measured by various methods such as **slit-lamp biomicroscopy** with an optical pachymeter, **ultrasound biomicroscopy (UBM)**, or **anterior segment OCT**. - This measurement is critical for assessing risk of **angle-closure glaucoma** and for intraocular lens calculations.

Question 313: Which of the following is the drug used for testing errors of refraction in a 7-year-old girl?

A. Atropine ointment (Correct Answer)
B. Homatropine
C. Tropicamide
D. Phenylephrine

Explanation: ***Atropine ointment*** - **Atropine** is a potent and long-acting **cycloplegic agent** that paralyzes the ciliary muscle, preventing accommodation during refraction testing. - Its long duration of action (up to 7-10 days) ensures complete cycloplegia, which is essential for accurate refractive error assessment in young children, as their strong accommodative ability can mask significant hyperopia. *Tropicamide* - **Tropicamide** is a shorter-acting cycloplegic agent (duration 4-6 hours) and might not provide sufficient cycloplegia for a comprehensive refraction in a 7-year-old. - While it causes mydriasis, its cycloplegic effect is less robust and sustained compared to atropine, making it more suitable for routine cycloplegic refractions in older children or adults. *Homatropine* - **Homatropine** has an intermediate duration of action (1-3 days) but is less potent than atropine for cycloplegia in young children. - It is sometimes used for cycloplegic refraction but is generally considered less ideal than atropine for very accurate assessment in children with strong accommodation. *Phenylephrine* - **Phenylephrine** is primarily a **mydriatic agent** (dilates the pupil) and has minimal to no cycloplegic effect. - It would not sufficiently paralyze the ciliary muscle to accurately determine refractive errors in a child.

Question 314: What does a visual acuity test primarily assess?

A. Ability to perceive light
B. Ability to differentiate colors
C. Ability to recognize shapes and details (Correct Answer)
D. Ability to detect contrast

Explanation: ***Ability to recognize shapes and details*** - A visual acuity test, typically using a **Snellen chart**, measures the sharpness of vision, specifically the ability to discern letters or symbols at a given distance. - It assesses the eye's capacity to resolve fine **spatial detail**, which is crucial for tasks like reading and recognizing faces. - This is the fundamental definition of visual acuity and what these tests are specifically designed to measure. *Ability to perceive light* - This refers to **light perception (LP)**, the most basic form of vision, indicating whether a person can detect the presence or absence of light. - While essential for vision, it is a much simpler function than what visual acuity tests measure and is assessed separately. *Ability to differentiate colors* - This is assessed by **color vision tests**, such as the Ishihara plates, which evaluate the function of cone photoreceptors. - It specifically checks for **color blindness** (e.g., red-green or blue-yellow deficiencies) and is distinct from the sharpness of vision. *Ability to detect contrast* - This is measured by **contrast sensitivity tests**, which evaluate the ability to distinguish objects from their background at various contrast levels. - While related to overall visual quality, it is a different aspect of vision than the ability to recognize fine details at high contrast.

Question 315: The most common cause of myopia is –

A. Altered curvature of the lens
B. Increased anteroposterior (A–P) diameter of the eye (Correct Answer)
C. Increased intraocular pressure
D. Changes in the vitreous humor structure

Explanation: ***Increased anteroposterior (A–P) diameter of the eye*** - This leads to the light rays focusing **in front of the retina**, which is the hallmark of **myopia** (nearsightedness) - The longer axial length means the eye's refractive power is too strong for its length - **Axial myopia** is the most common type, accounting for approximately **90% of all myopia cases** - Each 1 mm increase in axial length causes approximately **3 diopters of myopia** *Altered curvature of the lens* - While changes in lens curvature can contribute to refractive errors, they are a **less common primary cause** of myopia compared to increased axial length - These changes typically result in **refractive myopia** (curvature myopia), which accounts for only a small percentage of cases - More commonly associated with lenticular changes in conditions like early cataracts or keratoconus *Increased intraocular pressure* - **Increased intraocular pressure** is the primary characteristic of **glaucoma** and does **not directly cause myopia** - Elevated pressure can damage the optic nerve leading to vision loss, but it doesn't typically alter the eye's focal length - However, chronic angle-closure glaucoma can sometimes lead to secondary changes, but this is not a primary cause *Changes in the vitreous humor structure* - Changes in the **vitreous humor**, such as liquefaction or detachment, can cause symptoms like **floaters** or flashes of light - However, these changes do **not directly lead to myopia** or alter the eye's refractive power significantly to cause nearsightedness - Vitreous changes are typically age-related or associated with high myopia as a consequence, not a cause

Question 316: Snellen's chart is based on what?

A. Form perception (Correct Answer)
B. Light perception
C. Color perception
D. Contrast perception

Explanation: ***Form perception*** - The Snellen chart evaluates visual acuity by testing the ability to discern the **shapes and forms** of optotypes (letters or symbols). - Each line on the chart represents a specific visual angle, requiring the eye to resolve the **distinct geometric features** of the characters. *Light perception* - This refers to the ability to detect the presence or absence of light, a more basic visual function not measured by the Snellen chart. - Patients with **severe vision loss** might only have light perception, indicating a broad inability to resolve details. *Color perception* - This is the ability to distinguish different wavelengths of light, assessed by tests like the **Ishihara plates**. - The Snellen chart uses high-contrast black letters on a white background, and color vision is not a factor in its assessment. *Contrast perception* - While important for vision, contrast sensitivity is tested using specific charts with varying shades of gray, not the distinct black-on-white high contrast of a Snellen chart. - The Snellen chart assumes maximal contrast to isolate the ability to resolve the **form** of the letters.

Question 317: The refracting structures of the eye constitute a homocentric system of lenses. The refractive index of the cornea is:

A. 1.37 (Correct Answer)
B. 1.42
C. 1.31
D. 1.33

Explanation: ***1.37*** - The **refractive index** of the **cornea** is approximately **1.376**. This value is crucial for the cornea's role as the primary refracting surface of the eye. - Its high refractive power, due to the significant change in refractive index between air (index ~1.00) and the corneal tissue, is responsible for about two-thirds of the eye's total focusing power. *1.31* - This value is lower than the actual refractive index of the cornea. A refractive index of 1.31 would imply less optical density and reduced refractive power compared to the physiological value. - While within the broader range of biological tissues, it is not the accurate specific value for the human cornea. *1.33* - The refractive index of **aqueous humor** and **vitreous humor**, as well as **water**, is approximately 1.33. The cornea has a slightly higher refractive index than these transparent fluids. - While close, this value is primarily associated with the intraocular fluids and not the corneal tissue itself. *1.42* - This value is higher than the actual refractive index of the cornea and would suggest a greater optical density and refractive power than is physiologically present. - A refractive index of 1.42 is closer to that of the lens nucleus, which has a higher refractive index than the cornea to provide fine-tuning of vision.

Question 318: Which of the following is used to test color vision?

A. Amsler grid
B. Arden's gratings
C. Ishihara plates (Correct Answer)
D. Snellen chart

Explanation: ***Ishihara plates*** - **Ishihara plates** are a set of color plates used to diagnose defects in **red-green color perception**. - They consist of colored dots arranged in such a way that numbers or patterns are visible to individuals with normal color vision but difficult or impossible to see for those with color blindness. *Amsler grid* - The **Amsler grid** is a diagnostic tool used to detect **visual disturbances** caused by changes in the retina, particularly the macula, such as in **macular degeneration**. - It consists of a grid of horizontal and vertical lines, and patients are asked to identify any distortions, missing lines, or blurry areas. *Arden's gratings* - **Arden's gratings** are used to test **contrast sensitivity**, particularly in the diagnosis and monitoring of **optic nerve disorders** like glaucoma. - They consist of patterns of parallel black and white stripes of varying spatial frequencies and contrast levels. *Snellen chart* - The **Snellen chart** is primarily used to measure **visual acuity**, representing a person's ability to discern letters or numbers from a set distance. - It consists of rows of letters or optotypes that decrease in size, and patients are asked to read the smallest line they can clearly see.

Question 319: What is the typical axial length of the eyeball in a 3-year-old child?

A. 16 mm
B. 23 mm (Correct Answer)
C. 24 mm
D. 28 mm

Explanation: ***23 mm*** - The typical axial length of the eyeball in a **3-year-old child** is approximately **22-23 mm**. - Eyeball growth is rapid in the first few years of life, with the eye reaching about **90-95% of adult size** by age 3. - Adult axial length (23.5-24 mm) is typically achieved by **early adolescence** (13-15 years). *16 mm* - An axial length of **16 mm** is typical for a **newborn infant**, not a 3-year-old. - By age 3, the eye has undergone significant growth from the neonatal length. - This length would indicate **microphthalmia** or severe developmental delay if present at age 3. *24 mm* - An axial length of **24 mm** represents **adult eye length**, typically achieved in **early adolescence** (13-15 years). - While close to the 3-year-old measurement, this is slightly **longer** than typical for this age. - The eye continues to grow gradually throughout childhood beyond age 3. *28 mm* - An axial length of **28 mm** is pathologically **elongated** and indicates **high myopia**. - This represents approximately **4 mm beyond normal adult length**. - While some children may develop myopia, this degree of axial elongation would be considered **pathological** at any age.

Question 320: Which of the following is referred to as pseudomyopia?

A. Insufficiency of accommodation
B. Nuclear cataract
C. Spasm of accommodation (Correct Answer)
D. Presbyopia

Explanation: ***Spasm of accommodation*** - **Pseudomyopia** is characterized by a temporary shift in the eye's refractive state towards myopia due to an uncontrolled, sustained contraction of the **ciliary muscle**. - This persistent contraction causes the **lens to become more convex**, increasing its refractive power and making distant objects appear blurry, mimicking true myopia. *Insufficiency of accommodation* - This condition involves a **reduced ability to accommodate**, meaning the eye struggles to focus on near objects due to weakened ciliary muscle function or hardened lens. - It leads to symptoms similar to **presbyopia**, such as difficulty reading up close, and is not associated with a myopic shift. *Nuclear cataract* - A **nuclear cataract** is a clouding of the central part of the eye's lens (**nucleus**), which typically develops with age. - It often causes a myopic shift, known as **"second sight"**, where older adults may temporarily see better without reading glasses, but this is a structural change, not a muscle spasm. *Presbyopia* - **Presbyopia** is an age-related physiological loss of accommodation due to the **hardening of the crystalline lens** and weakening of the ciliary muscle. - It results in the inability to focus on near objects, requiring reading glasses, and is a normal age-related change, not a spasm.

Practice by Chapter

Physical Optics

Practice Questions

Geometric Optics

Practice Questions

Optical System of Eye

Practice Questions

Visual Acuity and Contrast Sensitivity

Practice Questions

Refractive Errors

Practice Questions

Accommodation and Presbyopia

Practice Questions

Optical Instruments

Practice Questions

Lenses and Prisms

Practice Questions

Retinoscopy

Practice Questions

Subjective Refraction

Practice Questions

Contact Lens Optics

Practice Questions

Wavefront Technology

Practice Questions

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