Light travels in a straight line (rectilinear propagation of light). This statement does hold if the medium of travel for light is

Light travels in a straight line, a phenomenon known as rectilinear propagation, specifically when it moves through a homogeneous and transparent medium [3]. A homogeneous medium is one where the physical properties, such as the refractive index, are uniform throughout [3]. If light enters a medium with a variable refractive index or passes through slabs of different refractive indices, it undergoes refraction, which is the change in the direction of propagation [1]. In inhomogeneous or non-uniform media, the speed of light varies at different points, causing the rays to bend rather than remain straight [3]. Fermat's principle confirms that rectilinear propagation is a special case occurring in homogeneous media [2]. Therefore, for the statement that light travels in a straight line to hold true, the medium must be both transparent and homogeneous [3].

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3 REFRACTION OF LIGHT > p. 145
2. [3] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3 REFRACTION OF LIGHT > p. 146
3. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 148