{"id":208164,"date":"2025-04-26T08:24:45","date_gmt":"2025-04-26T08:24:45","guid":{"rendered":"https:\/\/learnexams.com\/blog\/?p=208164"},"modified":"2025-04-26T08:24:47","modified_gmt":"2025-04-26T08:24:47","slug":"a-glass-block-in-air-has-a-critical-angle-of-49","status":"publish","type":"post","link":"https:\/\/www.learnexams.com\/blog\/2025\/04\/26\/a-glass-block-in-air-has-a-critical-angle-of-49\/","title":{"rendered":"A glass block in air has a critical angle of 49\u00b0"},"content":{"rendered":"\n

A glass block in air has a critical angle of 49\u00b0. What will happen to a ray of light coming through the glass hen it is incident at an angle of 50\u00b0 at the glass-air boundary? Illustrate with a diagram.<\/p>\n\n\n\n

The correct answer and explanation is :<\/strong><\/mark><\/p>\n\n\n\n

Correct Answer:<\/h3>\n\n\n\n

The ray of light will undergo total internal reflection<\/strong>.<\/p>\n\n\n\n

\n\n\n\n

Explanation:<\/h3>\n\n\n\n

In optics, total internal reflection<\/strong> happens when a light ray traveling inside a denser medium (like glass) hits the boundary with a less dense medium (like air) at an angle greater than the critical angle<\/strong>. Instead of refracting (bending) into the less dense medium, the light reflects completely back into the denser medium.<\/p>\n\n\n\n

In this situation:<\/p>\n\n\n\n

    \n
  • The critical angle<\/strong> is given as 49\u00b0<\/strong>.<\/li>\n\n\n\n
  • The light ray is incident at 50\u00b0<\/strong>, which is greater<\/strong> than the critical angle.<\/li>\n<\/ul>\n\n\n\n

    The critical angle<\/strong> is the minimum angle of incidence<\/em> in the denser medium at which the light ray is refracted exactly along the boundary (making a 90\u00b0 angle with the normal). When the incidence angle exceeds this critical value, no refraction<\/strong> occurs. Instead, the ray reflects entirely inside the glass, obeying the law of reflection (angle of incidence equals angle of reflection).<\/p>\n\n\n\n

    This can be better understood by Snell\u2019s Law:
    [
    n_1 \\sin(\\theta_1) = n_2 \\sin(\\theta_2)
    ]
    where:<\/p>\n\n\n\n

      \n
    • ( n_1 ) = refractive index of glass<\/li>\n\n\n\n
    • ( n_2 ) = refractive index of air (approximately 1.0)<\/li>\n\n\n\n
    • ( \\theta_1 ) = angle of incidence<\/li>\n\n\n\n
    • ( \\theta_2 ) = angle of refraction<\/li>\n<\/ul>\n\n\n\n

      At the critical angle<\/strong>, ( \\theta_2 = 90^\\circ ), so:
      [
      n_1 \\sin(\\theta_c) = n_2
      ]<\/p>\n\n\n\n

      Since ( \\theta_1 = 50^\\circ > \\theta_c = 49^\\circ ), Snell\u2019s Law cannot be satisfied with a real angle of refraction<\/strong> (you would get the sine of a value greater than 1, which is impossible). Hence, the light cannot refract into air and must totally reflect back into the glass.<\/p>\n\n\n\n

      Thus, a ray of light hitting the glass-air boundary at an angle of 50\u00b0<\/strong> will not exit the glass but will instead reflect entirely within the glass<\/strong>, following the law of reflection.<\/p>\n\n\n\n

      \n","protected":false},"excerpt":{"rendered":"

      A glass block in air has a critical angle of 49\u00b0. What will happen to a ray of light coming through the glass hen it is incident at an angle of 50\u00b0 at the glass-air boundary? Illustrate with a diagram. The correct answer and explanation is : Correct Answer: The ray of light will undergo […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[25],"tags":[],"class_list":["post-208164","post","type-post","status-publish","format-standard","hentry","category-exams-certification"],"_links":{"self":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/208164","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/comments?post=208164"}],"version-history":[{"count":0,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/208164\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/media?parent=208164"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/categories?post=208164"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/tags?post=208164"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}